Top Boy, Channel 4

One striking fact about Ronan Bennett's punishing four-part East End gang drama is that, so far, there hasn't been any sign of a policeman. No scruffy, down-at-heel detective with a chip on his shoulder, no thuggish Flying Squad heavies, and certainly no Wagner-loving aesthete who goes around quoting Elizabeth Barrett Browning.

Instead, Top Boy exists exclusively in its own sealed environment of violence, fear and dog-eat-dog ambition, where being sucked into gangland drug-dealing is the only option that bears any resemblance to a career ladder. Let's face it, how many kids can hope to get trials with West Ham United? As chief protagonist and aspiring dope potentate Dushane (a superb Ashley Walters) puts it: "I haven't anything to be except this." In the mostly black families living on high-rise housing estates, from where they can see the fat-cat towers of Canary Wharf sticking up like raised digits in the distance, the cycle of single mothers and children casually abandoned by their fathers is the anti-gift that keeps on taking, creating a demoralising vortex of welfare dependency and seething frustration.

But Bennett's feat is that he hasn't written Top Boy as a liberal cri de cœur or a Hackney social workers' manifesto. He has simply depicted a hideously believable environment and peopled it with brutally plausible characters, to the point where you could cry at the realisation that chunks of our dilapidated nation really look like this. Inventive colour grading and Brian Eno's coolly evocative music lend the project a cinematic weight.

At first I didn't think I was going to get on with it very well, since I couldn't understand anything anybody was saying for the first 15 minutes. Not only was all the dialogue delivered in gangsta/jafaican/ghetto/whatever, the sound also seemed to have been deliberately mixed into a sub-audible murk. The cavalry arrived in the heavily pregnant shape of Romford's finest, Kierston Wareing (pictured above with Malcolm Kamulete) as Heather, holding forth in strident Essex Girl as she embarked on a path of (she hoped) life-changing crime by nurturing a flat full of marijuana plants. This was at the behest of an insistent Vietnamese gentleman who promised her "very good price" for her leafy crop.

But Heather has been designed almost as comic relief from the dark end of the story, in which Dushane and his buddy Sully (Kane Robinson) take the decision to claw their way up the drug hierarchy on east London's imaginary Summerhouse estate.

This means they have to go into battle with turf-war rival Kamale (Tayo Jarrett) and make nice to menacing cockney kingpin Bobby Raikes (Geoff Bell, pictured left), who utters unspeakable threats in a voice reeking of jellied eels and blunt instruments.

The violence has escalated remorselessly from beatings and amputating fingers with secateurs to stabbings, live burials and shootings, but Bennett has managed to delineate some shades of grey in his characters. Dushane still holds onto some shreds of decency and can differentiate between necessary and gratuitous violence, and even granted his old mum's wish by going to church with her. Sully, by contrast, turns ever uglier as the stakes grow higher.

Their story is paralleled by the experiences of a group of kids a generation younger, revolving around 13-year-old Ra'Nell (Malcolm Kamulete). His single mum Lisa (Sharon Duncan Brewster) has been hospitalised - in a remarkably luxurious-looking facility - for depression, leaving Ra'Nell to fend for himself. He's given help and protection by Leon (Nicholas Pinnock, pictured right), apparently the only adult male for miles around who understands the notions of hard work, discipline and self-respect.

Leon is trying to keep Ra'Nell from being lured onto Dushane's escalator of crime, and we've seen it all go bad for Ra'Nell's white friend Gem (Giacomo Mancini), who was flattered by the gang-bangers' attentions until he suffered a near-death experience after being suspected of being a snitch for the enemy crew. It looks as if Ra'Nell has seen the light and has the strength of character to, er, just say no. In tonight's fourth and final instalment, we'll find out if they'll let him.