Seize the Day, Tricycle Theatre, London<br/>Category B, Tricycle Theatre, London

Obama is in the White House, why not a black mayor of London in City Hall, argues Kwame Kwei-Armah in Seize the Day. And it is the squeaky clean, council-estate raised, insurance broker turned personable television presenter Jeremy Charles who is hand-picked by the head of an equalities commission to run for office, having broken up a street fight on camera.

"Boris has just shown that you can walk in off the street and become mayor," says Howard Jones, played by Karl Collins as a smooth talking ducker-and-diver. Jeremy ticks all the boxes for the spin-doctors, although the Asian lobby would find fundraising for the campaign easier with one of their own up front. With 40 rising 45 per cent of Londoners from ethnic minorities, what can possibly go wrong? Human frailty, of course, in its often misleading manifestations – love, idealism, misunderstanding, self-preservation.

When Jeremy, played with endless charm by Kobna Holdbrook-Smith, decides to mentor streetfighting boy Lavelle, played with irony by the terrific Aml Ameen, he discovers behind the carefully arranged mask of indifference highly original ideas slickly articulated. On this relationship pivots the future of the mayoral hopes.

Kwei-Armah directs his own swiftly moving story with humour, teasing us with the occasional false trail and, just when sentimentality seemed to be sneaking in, springing one last surprise. He gets many a knowing laugh with references to Oona, Trevor, Bernie, Miss Abbott (who makes a brief appearance in one of the back projections which set each scene; Ros Maggiora is the designer). He sends up spin-speak – "We don't do Israel ..." – and is often simply wise: "It's not until you really, really know someone that you can look past their difference ...."

Seize the Day is, like Kwei-Armah's trilogy Elmina's Kitchen, Fix Up and Statement of Regret, concerned with the response of different generations of black people to the same problems, the tensions both between and within communities, and is the second of three plays in the Tricycle's Not Black and White season, that examine the roles in Britain of its ethnic minorities. Good-hearted and entertaining, with the help of vigorous and likeable performances, it is, overall, optimistic about diversity.

Pessimism, on the other hand, is the default position in HMP Thames Gate, where tensions run high in Category B, Roy Williams's contribution to Not Black and White. Saul (Jimmy Akingbola), prisoner-in-chief and king pusher, paces to and fro, a puma confined, vicious, all-seeing, all-powerful. Bobbing at his designer heels is Riz (Abhin Galeya), wired, eager to please and sustained by a total recall of films, whose plots, characters and set-pieces are more real than this half-life. Errol (Karl Collins again) has parole in his sights. Twitching his way to release, he pops pills at the level tolerated by warders who are expediently oblivious to both supply and demand. Containing the nascent rage and violence is a matter of "keeping the pressure", explains Angela (fiery Sharon Duncan-Brewster, also excellent as Susan in Seize the Day), soon to relinquish this wing to a by-the-book newcomer.

All the prisoners in Category B are black or Asian, a reflection of the disproportionately high number of black men incarcerated. Two are related; crime, or getting caught, runs in the family. Chandra (Jaye Griffiths) exasperated mother of "three youths, three different dads", wrestles with her wild, sexed-up cubs, holds down a job at Tesco, enrols for the Open University.

Category B takes on big themes with muscular ease: the ambiguity of compromise, the rehabilitation of prisoners, addiction, the carnal undertow of an all-male society, and the questionable worth of locking together men who are easily led, cross-fertilising destructive skills. A highly energised cast and Paulette Randall's brisk direction rattle these bars with urgency. Between the relentless slams, key-turning and steely footsteps of Tom Lishman's sound, a languid cello sings of a calmer, gentler world, out of reach. Occasionally the plot stutters – I could not believe in the affair between two colleagues – but the passionate performances give powerful voice to the flawed and the futureless against the severe backdrop of stairs, gantrys and closed doors.

'Category B' and 'Seize the Day' in rep until 19 Dec