IT WAS during the London première of Mandela: Long Walk to Freedom (Cert. 12A) that Zinzi Mandela was given the the news of her father's death. Since then, the world's media have been flooded with tributes to the former President of South Africa. Parallels have been drawn with the lives of Gandhi, Martin Luther King, and even Jesus. Mandela cautioned against hagiography, "unless you think of a saint as a sinner who keeps on trying".

This film, 16 years in the making (Mandela gave the South African producer Anant Singh the rights to his autobiography in 1996), does not shy away from the sins of its subject. His failings as a husband and father are laid bare, including an incident of domestic violence. Nevertheless, it leaves viewers in no doubt that they are in the presence of a giant.

Mandela, played by the British actor Idris Elba, towers above all who cross his path. At 6 ft 2 ins., Elba is just one inch shorter than Mandela. For younger audiences, it is a reminder that the elderly statesman was once a strapping young man, fond of sharp three-piece suits, with an even sharper line in seduction.

His stature is also symbolic. Throughout the film, he remains uncowed, radiating dignity, and authority, whether he is interrupting a cinema screening to tell the audience that they must "submit or fight", or demanding that his fellow prisoners receive long trousers. The effect is to portray Mandela as a giant, sublunary but extraordinary. As Jacob Zuma, his successor as President, said at the memorial event: "There is no one like Madiba: he was one of a kind."

It is, perhaps, because of this depiction that the most affecting scenes in the film are not those set on Robben Island, but those that tell the story of Winnie Mandela, whose response to the outrages perpetrated against her are both chilling and intelligible. While her husband is refined in the fire of incarceration, emerging, as Archbishop Tutu put it, a near-flawless diamond, she is shown growing hard as granite, full of rage, and thirsty for revenge.

When asked by Mandela how she bears police brutality, she replies: "By hating them. And don't tell me I am wrong. It keeps me strong." When she emerges from 18 months of solitary confinement, blinking in the sunlight, she warns the gathered press: "I am not young any more. And I'm not afraid any more."

It is a powerful performance by Naomie Harris, another British actor. Winnie Mandela has given it her blessing, describing it as the first time she has been truly captured on screen. Harris has described it as the hardest thing she has ever done: "I felt overtaken by her."

Throughout the film, she brilliantly captures the force of Winnie's dignity and hauteur, and the charisma that has secured her enduring popularity (Winnie was greeted with ululations at the première). "Don't touch me," she hisses at the guards who tell her she cannot take her baby into the court-house to hear her husband stand trial. It is hard not to experience a thrill when she emerges on the steps, after hearing the verdict, her head held high, her fist raised, as she cries "Amandla!" ("Power!"). The scene in which she is first imprisoned and screams for her children is terrifying.

"The circumstances she was in were primed to make her seek revenge and be full of anger and be violent," Harris told The Guardian. "I don't know how anyone else could react differently in the face of that kind of daily brutality that she suffered."

Although the film depicts the consequences of Winnie hatred, including her infamous speech about boxes of matches and necklaces, it is ultimately a sympathetic portrayal, clearly contrasting the paths of husband and wife, but explaining, if not excusing, the latter.

Towards the end of the film, Mandela observes that "What they have done to my wife is their only victory over me." The pathos of this admission is affecting because of the contrast with the early scenes of their courtship, which are electrifying, a beautiful, boisterous young man meeting his match.

Some may take issue with the film's representation of Mandela's faith, or rather, lack thereof.

"I don't see your God caring for your people," he tells his first wife, Ethel. "It seems to me that he is looking after the bosses."

When he does refer to God again, it is a cynical attempt to win de Klerk's co-operation, by repeating back to him what he has been told is the President's own conviction: that God is calling him to "save the people of South Africa".

Telling the life story of Mandela in a feature film is arguably an impossible task, and Long Walk is not particularly inventive. Instead of concentrating on a particular episode, as Lincoln did, or experimenting with chronology, as Amadeus did, the writer, William Nicholson, has opted for a straightforward linear narrative, interspersed very occasionally with contemporary footage.

It is a well-paced film, nevertheless, which conveys the moral authority of its subject, and, in devoting attention to Winnie's story, paints a sobering picture of what might have been, had hatred proved stronger than the desire to live free from fear.

On release from today.