There's no nice way to put it: the most powerful scenes in Blood Diamond are as compelling to watch as they are sickening. Young children, torn from their families by armed thugs, are conditioned first to love guns before being sent into villages to kill unarmed people as though they are playing a video game. They are even given cartoonish nicknames such as "The Master of Disaster" and "See Me No More" to heighten their sense that murder is playtime.

It's part of the brutal but effective process of desensitisation the child soldiers undergo in Blood Diamond, the confronting, serious-minded, self-consciously controversial film from director Ed Zwick ( Glory, Courage Under Fire). In a film climate stuffed with empty-headed blockbusters and heartfelt, critically adored independent tales, Blood Diamond attempts something truly ambitious - a mainstream, big-budget, star-driven film with a conscience. It succeeds admirably.

Set in the small African country of Sierra Leone in the 1990s, the film explores the dark world of the blood diamond trade, where rough diamonds are used to fund militia rebels at war with their government.

We see events through the intertwining stories and converging interests of Danny Archer (Leonardo DiCaprio), a diamond-hungry former soldier; Maddy Bowen (Jennifer Connelly), the standard-issue movie journalist there to lubricate story exposition; Solomon Vandy (Djimon Hounsou), a local man who finds a diamond the size of a mafia wife's bling; and his son Dia (Kagiso Kuypers), who is captured by rebels and brain-washed into becoming a trigger-happy child soldier.

The portrayal of brute violence in dramatic films has always presented filmmakers with a lattice of trip wires. How far do you go? At what point does it become gratuitous? What degree of shock can you inflict on your hapless audience without making them regret how they didn't go see that nice romantic comedy with Cameron Diaz instead?

Wisely, Zwick, working from a screenplay by Charles Leavitt, deals with the matter by restricting the more explicit moments of violence to horrific flashes: one child executes another as target practice; babies are murdered; arms are jokingly hacked off with the cruel taunt, "long sleeve or short sleeve?" It is the detailing of how casually the violence is perpetrated, especially by the children, that gives the film its dramatic force, not bathing the viewer in torrents of blood.