Mia Farrow blinks in the pale, wintry sun as she fingers a trio of necklaces around her neck: a star of David, a crucifix and a Koranic leather pendant. “You see – I’ve got all bases covered,” she smiles.

The Hollywood actress turned peace campaigner was in London last week to launch a fund to support the survivors of the ethnic cleansing in the Darfur region of Sudan. She arrived amid the furore over Gillian Gibbons, the primary school teacher imprisoned for allowing children in her class to call a teddy bear Mohammed.

Gibbons had then been in prison for five days. As far as Farrow was concerned, there couldn’t be a better illustration of the “insanity and cruelty” of the Sudanese government. “And this is only one woman,” she said. “Imagine what might happen to her were she out of the public eye and were she not white.”

After a high-level cross-party intervention by Lord Ahmed and Baroness Warsi, both of whom flew to Khartoum to plead Gibbons’s case, she was released. “Of course we are all very relieved that Mrs Gibbons was allowed to safely return home,” Farrow says. “But isn’t it interesting that one white woman and a teddy bear have captured more attention than the atrocities of the past three years in the Darfur region of Sudan? Now we should turn our eyes to the plight of the millions of men, women and children in Darfur who are now living in camps where they are not safe.”

The leather pendant Farrow wears was given to her on her first visit to Darfur by a woman whose baby had been torn from her back and killed by the Sudanese militia, the Janjaweed. Farrow’s huge eyes widen. “She clasped my hands and said to me, ‘Tell people what is happening here in Darfur or we will all be slaughtered by our own government.’ She insisted I wear the pendant to protect me. I, who could offer her no protection!”

The Fund4Darfur, which she is starting with the Aegis Trust, will go to help such women. It would be easy to mock: celebrities are constantly wringing their hands and telling us how much they care about the ills of the world. But Farrow, to give her credit, has put her ideals into practice.

She adopted 10 children from deprived parts of the world years before Angelina Jolie made it fashionable. And she knows her stuff, having been to Darfur seven times.

She first became interested in 2004, after the first spate of killings took place, on the 10th anniversary of the Rwandan genocide. “I had this awful feeling of failure about Rwanda,” she says. “Your government and my government did nothing, and in America we were sitting around watching the OJ Simpson trial.”

She read a New York Times article about the ethnic cleansing in Darfur and thought: “Are we to conclude that the words ‘never again’ apply to only white people?” On her first Darfur trip she saw the full horror of “the first genocide of the 21st century: a completely helpless civilian population being killed by its own government and its proxy militia because they were not Arab. It was beyond my comprehension”.

About 300,000 people are believed to have been murdered in Darfur: more than 2m have been driven from their homes. Everywhere Farrow went she met men, women and children and heard stories “which bore a numbing familiarity. The people were always attacked in the early morning; first the sky is full of bombers and helicopters while people prepare breakfast, or go to the fields, or pray. Then you hear how the Janjaweed [Arabic for devils on horseback] follow on camel or horseback, shooting people as they flee. The women and children are often raped and mutilated too”.

As Farrow tells the stories of the people she met, she shows many incredible photographs she’s taken. Some are of the Janjaweed carrying rifles and rocket-propelled grenades.

She encountered them at first hand a year ago, while driving along the Chad-Darfur border. First she saw women and children, “terrified, dazed and screaming” as they ran away from the attackers. Then, as her vehicle pulled into a clearing, Farrow saw the armed bandits, carrying AK47s, their heads swathed in scarves, riding towards the vehicle on camels and horseback.

She describes how she jumped out of the vehicle and began fumbling with her camera. “I’m bumbling round like I’m Lucille Ball,” she says, “throwing my little camera around, saying, ‘Look, it’s not a gun! Whoops! Oh! I’ve taken a photograph!’ ” Why did she take such a risk when she knew these men were killers? “I felt it was important to tell that this is what’s happening to the people of Darfur on a daily basis,” she says. “To verify; to bear witness.”

Farrow was travelling alone on that trip, or, as she puts it, “independently”. On others she has been accompanied by Ronan (formerly Satchel), her son by Woody Allen. Mother and son are touchingly close. Today this prodigiously clever young man is 19 and at Yale Law School. “There are limits to where I bring him,” his mother says.

As she scrolls through the files on her laptop we reach photographs of her children, some with babies: her grandchildren. “This is my life,” she says, almost to herself. “My crazy life.”

At nearly 63, she still looks waif-like and ethereal and is dressed in a sort of Peter Pan outfit: grey waistcoat over black trousers and white linen shirt. She has a porcelain skin, an ever so slightly little girl, lispy voice and a sweet smile and is good company . . . but you sense she has an inner armour of steel.

The daughter of Hollywood royalty – her father was the director and writer John Farrow and her mother the actress Maureen O’Sullivan – she had a “magical childhood” that ended abruptly on her ninth birthday when her legs buckled beneath her. She had polio and awoke the next day to hear her family praying she wouldn’t die.

She was brought to hospital, the house sold and everything in it she’d touched destroyed. “It was as if there was a death in the family,” she says. “Everyone was afraid of me – even my mother.”

Even though she was the child of privileged parents growing up in one of the richest parts of the world, contracting polio “made me feel like a pariah and left me with the desire to relieve suffering. Today I still feel I’m in a lifeboat pulling in all these people in the world in pain and distress”.

Farrow was 21 when she married Frank Sinatra, who was 51 and a friend of her father’s. Their divorce papers were served on the set of what is probably her most famous film, Rosemary’s Baby. In l970 she married André Previn, with whom she had three children and was terribly lonely in a large, damp house in Surrey; her husband was always away conducting. She began adopting children, including three daughters from Asia: Daisy (also known as Summer), Lark and Soon-Yi.

But it is Farrow’s 12-year relationship with Woody Allen for which she is most famous, or “notorious”, as she says herself: 15 years ago she discovered some nude Polaroids of Soon-Yi, then aged 21, that had been taken by her stepfather, Allen, who was then 56. A vicious custody battle ensued amid accusations of child abuse. Today Soon-Yi is married to Allen, and Farrow says quite simply: “She can’t be my daughter any more.”

For years she and the children lived in a rambling farmhouse called Frog Hollow, in rural Connecticut, complete with horses, rabbits, cats and dogs. Today, she says, most of her children have fled the nest, leaving only one daughter, Quincy Maureen, 14, who’s only intermittently at Frog Hollow, having opted to go to boarding school.

Farrow still acts – last August, between takes of a Luc Bresson film, she sent a message to Omar al-Bashir, Sudan’s president, on her BlackBerry, offering to swap places in prison with Suleiman Jamous of the Sudanese Liberation Army, who was in urgent need of stomach surgery. “Multitasking!” she grins. They didn’t accept her offer but freed the SLA leader anyway.

Whatever you think of her motives, most friends and critics think Farrow is playing her most important role to date. It was she and her son Ronan who sounded the alarm about China, a major trading partner of Sudan, hosting the 2008 Olympics.

In an article for the Wall Street Journal, they dubbed the Games the “Genocide Olympics”. Farrow says: “China is underwriting Sudan’s genocide in Darfur,” adding that Steven Spielberg, who has been hired to orchestrate the opening and closing ceremonies, should cut his ties with the event “or risk going down in history as the Leni Riefenstahl of the Beijing Olympics”.

And she speaks of the plight of the Darfuri children and the constancy of their mothers with a movingly heartfelt admiration. “They’ve taught me to be a better person,” she says. “The women of Darfur are the most courageous I’ve ever met. From them I’ve learnt what it is which cannot be taken away: friendship, love, courage, generosity, good parenting. I’ve seen the very best of human beings in the very worst of circumstances.”

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