This is the East End of London on a Thursday night, and the city is just warming up. The nightclubs in Shoreditch are opening their doors. Exotic smells drift from the curry houses and bagel shops at the top of Brick Lane. Crowds flood out of the local mosque - once a synagogue, and before that a Methodist chapel and a Huguenot church - flocking to their iftar (fast-breaking) meals.

The traditional sweetshops are busy, too, young men crowding to buy ladoo, jellaby or perhaps dates imported from the Gulf. Street prostitutes - 'toms' as the police call them - ply their trade in the back alleys around Brick Lane or behind the trendy cafes and restaurants which have sprung up in the newly named 'Banglatown' (Whitechapel and Spitalfields) during the past few years. Even Gordon Ramsay hosted his Hell's Kitchen here, in the famous Truman Brewery, while tourists on their Ripper tours and bohemian young whites rub shoulders with the new City folk moving in.

Looming above all, Canary Wharf shines. The chrome of the City is barely a mile away. We are in the heart of Europe's most powerful metropolis. Yet we are also travelling into the abyss.

*

'What you up to? Kotchin' [chilling out]?

''Yeah, kotchin'... officer.'

The slang is 'Banglish', the curious mix of English and Bengali which on the street is often mixed with Jamaican patois and prison slang. The unmarked police vehicle has pulled into a small alleyway. It is pitch black. I can just make out a sign above us, offering salvation in the evangelical church behind.

The window slides down; a face pokes out from a red Ford Fiesta, bleary-eyed, confused, resentful. A haze of smoke wreaths the occupants: two young men, Bangladeshi, both with sculpted, thin beards, one with a baseball cap pulled low over blanched features. On instruction, they step unsteadily from the car. A third, a young woman, her bleached hair clashing with dark skin, is frozen to the centre of the back seat. The two plainclothes officers from the Robbery Task Force flash a torch inside, briefly illuminating the foggy interior.

'What's this, then?'

Ken, a close-cropped Pc who enjoys Aussie Rules football and marathon running, emerges with a crack pipe in his hand. It is a simple Coke bottle, through which an empty biro has been pushed to form the pipe stem. It steams in the night air. The driver shrugs.

'Twenty quid will buy you a wrap [of drugs],' Ken tells me. 'They usually come wrapped in different coloured plastic, depending whether it's rocks [crack] or brown [heroin]. You find this stuff on plots [drug stashes] all over the place.' But the pipe is of little interest, quickly discarded. He turns to frisk the youths, who are staring sullenly in my direction, one of them loudly proclaiming he will 'sue you all', while Dave, Ken's partner, calls through to Bethnal Green police station with a situation report. The car has been connected to a series of robberies, and the two policemen are part of a 16-man squad combating chronic street crime in London's Tower Hamlets. Most youths recognise these unmarked cars with ease. A cat-and-mouse game usually develops as they cruise the tiny alleys and parks, travelling through the high- and low-rise estates, searching for troublemakers.

Gang members I met while travelling with the 'Robbery Squad', as it is known among the Bangladeshi youths hereabouts, were keen to tell me of the 'massives' and 'posses' they hung out with - 'We're Shadwell Massive!' one group proclaimed, as their friends were frisked by police - protesting to my face that they were indeed serious gangsters, these barely-men with their first beards, faces cloaked in darkness, eyes shining in the glow of reefers.

*

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There are worlds here, new and old, that rarely mix, the newcomers affluent, drinking in gastropubs and bars, their preferred drugs cocaine or 'pills' (ecstasy), failing to glimpse life on the estates so close to their gated communities and city apartments. Gentrification gathers apace: the East End now hosts 40 per cent of all London property development. Yet it is also one of the most deprived areas in the UK.

Nearly half the families here now live below the poverty line; it is also home to the largest concentration of Bangladeshis in the country. They form about 40 per cent of the borough. Everyone seems related, through extended family and village networks that have travelled over from rural Sylhet, bordering Assam in northern Bangladesh.

One hundred years ago the East End was a ghetto, the centre of the Jewish community in England. Now the azan, the call to prayer, sounds out from the towering East London Mosque - London's oldest - and the tales are of Shahjalal, the Yemeni warrior-pilgrim who brought Islam to the Bengalis. I had watched, entranced, as 15,000 men prayed on the roads surrounding the East London Mosque in Whitechapel, during the opening of its huge sister building, the London Muslim Centre. An imam from Mecca led the prayers, the faithful crying 'Allah Akbar!' ('God is Great') as they laid their prayer rugs out on the street. The police had to shut down the entire area. I wondered if those in the City, but a stone's throw away, were aware of these events, how an Islamic identity was building in Europe's financial heart.

What is most evident about these streets now is that gang culture has taken root. Usually composed of young men between 14 and 25, the gangs 'claim' patches of territory, estates. Most are fiercely territorial. Step into the wrong patch of land, wander into the wrong street, and violence can be swift and brutal. Often it is their own - second- or third-generation Bangladeshi young men - that pay the price.

And a large proportion of Bangladeshis are young. The birthrate is high. Teenage boys find themselves on the street, local authority housing insufficient for their families, particularly where girls who reach puberty need a room of their own (in what is still largely an Islamic society). Fathers who came here alone many years ago have spent those years sending money back 'home'. Many bought property in Bangladesh and always intended to return. Neither as highly educated nor perhaps as aspirational as some other South Asian communities, they had ambitions limited to either the rag trade - now collapsed - or the restaurant business (about 80 per cent of all curry houses are Bengali-owned). Arranged marriages remain common, too, the (invariably) young couple often expected to move in with the groom's parents. Youngsters can find themselves trapped in an identity vacuum, belonging neither to the secular West nor to their parents' rural Islamic East.

Cycles of retribution are common and pitched battles have taken place, hundreds strong. Drugs often fuel the problem, leading to endemic street crime. Crack cocaine and heroin have flooded the East End, Bangladeshi 'runners' merely footsoldiers in a wider war. With unemployment high and educational achievement among boys poor, narcotics are the currency out of the ghetto.

Ironically, the youths I talked to spoke of their pride in 'beating the BNP' in its former East End heartland; how they formed the 'Bengal Tigers', one of the first gangs, to see off skinhead attacks. Yet the very groups protecting the area from vicious 'Paki bashing' in the 1970s and 1980s (Altab Ali Park near Brick Lane commemorates one such murder victim) now provide a perfect network for the drug dealers.

'Each estate has its own "firm",' a local probation worker tells me. 'The firms start selling drugs and a lot of them are into fighting; it's all about their territory. That's where getting your "rep" comes from.'A survey has revealed that nearly three-quarters of all Bangladeshi primary-school children rated fighting as a friend's most important attribute.

On Roman Road ('the Roman'), a haunt of the Kray twins in Bethnal Green, I heard shopkeepers and youth workers complain of the latest group, the 'Deadly Alliance'. Nearby, a huge luxury apartment complex is rising on the banks of the Grand Union Canal; an arts pavilion hunches into the side of a newly sculpted nature reserve in one corner of Mile End Park. Further north sits Bow Wharf, with bars and comedy clubs, and then the fashionable Victoria Park, its approach guarded by a gastropub. It is strange how such worlds sit side-by-side.

Ali is in some ways typical of many of the young men who live here. A former Somali gang member (the Somalis are another prominent minority group, also Muslim), he has run with many Bengalis. He worships American hip-hop stars, wears a prominent gold chain, expensive black leathers and a leather baseball cap. His fingers are elfin-long, his face and body thin and elegant, his skin darkest ebony. He carries the latest mobile phone and speaks in a confusing, fast rhythm, blending Cockney, Jamaican and street patois. He tells me about chewing khat, the mild stimulant preferred by many Somalis, then coming down on 'weed' [cannabis], before proclaiming his belief in Islam and his attempts to pray regularly. Unlike many Bangladeshis, he has seen war close up and talks nonchalantly of 'some serious boom-boom', or of a cousin arrested for selling sub-machine-guns right here on the street.

'It's normal to get into fights here over drugs or a little local "beef". Maybe a girl. It's not racial, it's territorial,' Ali says. 'Everyone smokes [cannabis] but some of these kids are into it [heroin and crack] hardcore.'

The Metropolitan Police feels that the crime problem among South Asians in London now warrants the creation of a special unit, similar to Operation Trident (tackling gun crime among the black community). The Met's Specialist Crime Directorate Assistant Commissioner, Tarique Ghaffur, warns of 'crime-ridden ghettos' if the idea is not taken up.

As Chief Superintendent Mark Simmons, the police area commander, points out, 'Unemployment is high. Canary Wharf is an anomaly. This is one of the worst half-dozen boroughs in the country in deprivation terms, and it therefore has all the problems that you'd expect. The area has been called the heroin capital of the UK. We have significant issues with class-A drugs.'

One of the biggest issues police and locals face is anti-social behaviour - low-level crimes which, if unchecked, can lead to other problems or make life hell for communities. Simmons says his teams are making strong efforts to reach out to community leaders, and relations do show some sign of improvement, despite past mistrust.

There are also a significant number of racist crimes reported. 'A very substantial proportion of racist incidents are Bengali suspects with white victims.' Perhaps with the demise of the Krays and the old villain networks, serious crime should have left the East End. Yet the world of the gangs can lead far beyond anti-social behaviour or mere street battles.

Two months out of prison, Rasul, a 24-year-old convert (or more technically, 'revert') to Islam, is undergoing a pre-Ramadan fast. His conversion has taken place 'inside'. He has served a four-year sentence for conspiracy to supply class-A drugs. He now volunteeers as a mentor on a youth project.

Like many Bangladeshis I meet, he is short. He sports a wispy beard. His skin seems almost translucent; his voice is a bare whisper, the thick Cockney accent mixing with a shyness that belies his past. For Rasul was a 'top shotta' - a major drug dealer. He made his living running street soldiers, paying them regular wages, ripping off other dealers and organising protection rackets. His stories are truly frightening.

Drugs brought him wealth, but also sorrow. 'You can't relax, you get paranoid, always expecting the rip-off, or police, or other problems. My son died too. I stopped caring about it any more.' Rasul ended up as a heroin user, breaking the cardinal street rule, 'don't get high on your own supply'. From there, his downfall was sealed.

We walked the streets of Bethnal Green together. Rasul seemed full of remorse, though I imagined he would struggle with a life of mundanity. A youth approached us in a side street: the two men swapped an elaborate series of Arabic greetings, before touching the areas above their hearts and ritually embracing. The youth pushed back his baseball cap and made Rasul take his mobile number.

'I'm not going to store that,' he confided in me, as we moved off. 'I just don't want that kind of thing no more.'When I pressed him, he admitted he had used the other guy - working as a runner for a Yardie (Jamaican) drug gang - to help set up a job, ripping off the Jamaicans ('it was balaclavas and all that, crazy') and stealing their supply. The youth was kidnapped in response, held until a deal was negotiated between the two groups. The Jamaicans were more concerned with retrieving their two mobile phone numbers, used as their main dealing lines, than anything else: 'Worth about 30 grand,' Rasul said, 'much more than the drugs.' For years he had lived a life in which retribution seemed to hover around every corner.

I asked him about his childhood. I wondered how this mild-mannered man had become such a notorious gangster. 'I had seven sisters, two brothers,' he started. 'It was a crowded time. A three-bedroom place. It just began from there. The arguments. We had to move about from hotel to hotel; mum didn't get on with my dad, things like that. Different hotels all the time, moving about, because my dad and mum didn't sort out their arguments. My mum couldn't hold back her anger. We'd come back after a few weeks or months, 'cos he'd calm down a little bit.'

His father eventually moved out, returning to Bangladesh and taking a second wife. 'I was the eldest then. I had to do everything for my mum.' Rasul sighed, struggling at times to express himself. 'It started from school, yeah, just bunkin' off. All Bengali boys, Asian boys. You just go in there, do what you like. Boys smoking in the classroom. Teacher drunk.' He had already developed a reputation as a fighter, even from primary school. 'People just respected me, I guess. And I just started taking advantage of them, y'know? Just taking people's money. Then I got introduced to the older lot. That's when the drugs came in. I just thought, yeah, bit of money.'

From 'chillin' and rollin'' as he puts it, Rasul became a gang leader, taking part in what he readily admits was 'honky [white] bashing' - something I had heard from older whites, but so far dismissed. A gang formed and 'we started attacking pubs. A man come out of the first one, he got done. It became a white and Asian thing, we thought we'd attack anyone we'd find. We were looking for trouble, 'cos they were attacking us. We were so narrow-minded then! One of my friends got sliced in Brick Lane by white fellas. There were a lot of fights back then. We couldn't get along with them and they couldn't get along with us. So we started attackin' pubs, anything, tryin' to scare them off.'

'It went outta control,' he said. 'Everyone started fighting, like, with the blacks as well and some of them got too big for the head and doing things. We done planned attacks on the pubs and like, but then everyone just wanted to do their own thing. It becomes like sex.'

They would travel across London, thieving and robbing in the West End - just like the Krays and others before them. But Rasul had got involved in drugs, taking him away from the world of the territorial gangs. 'That network broke. People started going to prison.'

He admitted, 'I've done robbery before. On restaurants. People would come and tell me that that guy's taking 15 grand from here to there; I just would go out there to do the job.' He talks of influence over certain people, his threats and protection rackets, robbing businessmen and then other drug dealers. His eyes seem to light up when talking of the past; it's only when we pass a burnt-out business - a business he once targeted and threatened - that the sorrow enters his voice once again, and he falls silent.

'They just think about money, making money, that's it. I don't see it as easy money, though. Always watching their back, always thinking someone's round the corner, or the police.'

*

It is a few days before Ramadan, and I am speaking to Hamida Begum, 23 ('begum' is a common nickname, meaning 'woman'). She is tiny, sweet-faced, her smile radiant but nervous, a cigarette clamped almost permanently in her fingers. I have to lean in close to hear her. Dressed in Western clothes, it takes her a while to speak with confidence. Once in her stride, her passion, her pent-up frustration, is hard to stop.

It has taken me the best part of a year to get close to any women in the Bangladeshi community. Even now, when visiting people's homes, I have to stay in separate rooms to mothers or sisters, my presence potentially forbidden because I am not mahram (blood family), the concept of purdah (governing contact between men and women) still strong. 'To be honest, all the drug dealers I've been hangin' out with, all the gangs, yeah, they've all got a sob story. They're not happy deep down sort of thing. Yesss,' she pronounces. 'Most of them turn to Islam afterwards, innit? They come clean. They're total losers though. They've got kids with prostitutes and everything, they're all takin' it [drugs]. Even people who go to the mosque.'

Though I would say, that boyfriend I was with, he never tried to make me do drugs,' she adds rather quickly. Hamida and her boyfriend would have arguments, she telling him he couldn't supply a 12-year-old that had called for drugs, the boyfriend saying the kid would get the drugs from someone else anyway. 'I said, at least you're not selling it to him. Some days when I used to nag him, yeah, he wouldn't sell it. Three times a boy called him up from the hospital, saying he wants it. But he's dying of asthma, got pipes in 'im and all that and he wants it, and he'd say, "No, I can't give it to you." But he's begging and begging. He said, "Look, I'll go someone else anyway." It's just 20 quid.' She falls silent.

Hamida went out with a drug dealer, a 'top shotta' just like Rasul. Raised in a strictly Islamic home, she attended religious school - a madrassa - where she was fully veiled. Hamida has rebelled against her background and is stinging about the hypocrisy within the community. Her father, too, has illegitimate children, she says, but as long as he can show a respectable face to the community, and to the elders in the mosque, that's all that seems important.

'I was caught with a gun,' she says a little sheepishly, voice going low. She giggles, embarrassed. 'I was climbing out of the window, trying to run away from home that night [her brothers had forbidden her to leave] and, um, I had the gun in my pocket. If I ran away I thought they might get the police in or whatever and go through my stuff, find the gun, and it would be better off if I took it with me and got rid of it. So that's what I did.' Loaded? 'Loaded. I didn't get that far. I tried to climb out of the third floor. I fell. Broke my back and everything. Ended up in the hospital.' She sighs, looking out of the cafe window for a moment. 'I was supposed to be in a wheelchair. But I'm OK now.'

As a friend later explained, 'Most dealers use their women to hold things for them. If they get busted, the police aren't gonna find nothing in their house. What normally happens is that if a guy's got a beef goin' on, he'll call his girl and say, "Listen, I need ya to bring my thing here" ' - he slaps his hands - ' "blah de blah de blah." The girl will bring it, he'll send her back and then he can go and do his business. And that's how they access the things they need.'

Amid all this sorrow and pain the community is fighting back. It has helped many young people find new direction and new life. Khalid is part of this work. He is a soft-spoken undergraduate, a polite young man. Once feared on the streets for his charisma as a gang leader, he now works for a project called the Rapid Response Team (RRT), set up by the Tower Hamlets council.

While Khalid's colleagues deal with prevention and mentoring, using a series of mobile youth centres or by talking with community members, he is at the sharp end of the problem. At a moment's notice he might be called out by police or residents to help break up a fight or tackle anti-social behaviour. The young men look up to Khalid because he was once one of them. You can see it in their greetings as he approaches. His presence can calm things. But it isn't easy. When we first talked, he was dealing with the aftermath of a near-decapitation. The next time it was a shooting.

'These kids, they're out of touch with our culture: the elders have lived one part of their life abroad and don't understand the younger Bengalis around them,' he says. Many parents still want to arrange marriages for their children or try to send them back to Bangladesh if they misbehave. 'We are tight, close. But we are also confused.' He tilts his head and sighs. 'I have my father and mother's culture to uphold, and Western culture too. I turn on the TV and I hear about Islamophobia. How do you balance all of these factors?'

Though the team deals with gang violence, anti-social situations can often be worse. As Khalid says, 'We've had an incident where there's a single mother and she can't walk up to her flat because there are 10-15 boys who hang around and urinate on the stairwell. Now for her that behaviour is far worse than a full-blown fight.'

The gangs are almost like a family. Because of overcrowding, and lack of identity, kids often spend a lot of time on the streets. 'Even our own aunties and uncles [Bangladeshi elders] have been mugged...' mentions one of Khalid's colleagues, sadly. 'There's a huge identity crisis with the youngsters here. Often you find they don't know who they are. When they know who they are, they can stand on their own two feet and feel proud.'

Islam has a part to play, too. It has transformed the life of Abu Mu'min, who was brought up in these streets, and is now a father and social worker. 'For most of the people you meet, those who've come out the other side, faith has been central,' he says. 'I'm sure a lot of young people are confused, but they want to get married some day, not remain the same. There's so many pressures, mental-health problems too. You need spirituality: you look at the Creator and you feel peace.'

The 34-year-old Mu'min works for BLYDA, the Brick Lane Youth Development Association. Situated in an old school building just off Vallance Road (another of the Krays' old stamping grounds), it has respect on the street: it was founded by ex-hardcore offenders. One of its first acts was to mediate between a huge gang dispute involving hundreds of men and youths from Poplar and Brick Lane. 'This was back in 1997. We'd have large-scale fights taking place over the area. People were getting seriously injured. We're talking about axes, knives and hammers, and some of the young people are still serving prison time as a result. There was no hope. The police and the community didn't know what to do.'

BLYDA came in and organised a peace conference at the only neutral ground, which was the East London Mosque. We had 150-180 on each side. They were preparing to get guns. Our members spoke to them and explained how it was important to work together. We managed to calm it all down.'

But the wider problems persist. With the azan drifting with the drizzle across the Isle of Dogs, I watch as a full eight-man complement of the Robbery Task Force breaks down another door. Another raid, another bust: two men and a woman are led out in handcuffs. The noise is horrendous, booming across the estate. Not a single door opens, not a curtain twitches. This is just another night in the East End.

Looking down the black length of the Thames, Docklands glitters, tantalising, promising opportunity and riches. In the City institutions nearby, salaries on average sit at £60,000. Few Bangladeshis work in either. Their forefathers were known as the men of the seven seas and 13 rivers, crossing oceans of water to come here for a new life. Many secretly hoped to return. What life now beckons for their children?

This article originally appeared in the Telegraph Magazine © 2005.