'Melting pot' is the hackneyed cliché normally wheeled out when describing the rich diversity of Los Angeles' cultures, creeds and races.

But writer-director Paul Haggis doesn't see it that way. To him, LA is less a melting pot and more a simmering cauldron scorched by an unforgiving Californian sun.

Just about keeping the lid on the volatile mix is an administration - law enforcement, judicial system and local government - that is riven with racism and entrenched corruption.

Using a complex but comprehensible web of fraught relationships, chance encounters and random circumstances, he provides a snapshot of a city strained to breaking.

Jaded detective Don Cheadle is an honest cop who has fought his way through the ranks but is now wilting against the onslaught of establishment palm-greasing.

Matt Dillon's traffic cop subjects a yuppie black couple to racially-motived harassment with a creepy sexual edge when he can't get his suffering dad past an officious black medical centre clerk.

At other end of the scale, district attorney Brendan Fraser and Sandra Bullock put up the barricades - new locks, hi-tech alarms - when they are car-jacked by a couple of black thugs.

Reflecting on his predicament, Fraser complains that the robbery is going to cost him either the black vote... or the law'n'order vote.

All over the City of Angels, this sort of friction - no respecter of race, class or wealth - threatens to erupt as the threadbare social fabric of the sprawling metropolis develops tiny tears and rents that threaten to become chasms.

Haggis - who scripted the Oscar-winning Million Dollar Baby - has taken a brave stand in recognising that white fears of black crime and black suspicions of white police corruption aren't always irrational.

Appearances can be deceptive - no-one is entirely guilty and there's isn't a single soul who's completely innocent. You just have accept it and get on with life.

Ambitious and caustically scripted, things only go a little fluffy in the final reel, which merely serves to underline how high the bar has been raised before we get here.

Essential viewing.

Tim Evans