My first visit to Darfur was in 2004. It changed the way I needed to live my life. I have just returned from my seventh trip to the region. I don't think I have the words to adequately represent what I have seen and heard there.

Incomprehensibly, it has now been more than four years since the killing began. Some experts believe half a million human beings have died thus far. Others bicker about the exact death toll - as if it makes a shred of difference to how we must respond.

Only the perpetrators dispute that hundreds of thousands of innocent men women and children have been killed, in ways that cannot be imagined or described. It is all the more appalling that we cannot know - that no one is yet able to count the dead. And the dying continues.

We can, however, know with certainty that more than four million people are dependent on food aid because their homes, villages, and the fields that sustained them, are ashes now. We also know that two and a half million human beings are struggling to exist amid deplorable conditions in squalid camps across Darfur and eastern Chad. I am a witness to their suffering.

The stories of those who survived the attacks are numbingly similar. Without warning, Antonov bombers and attack helicopters filled the morning skies and rained bombs upon homes and families as they slept, as they played, as they prayed, as they tended their fields. Those who could run tried to gather their children and fled in all directions.

Then the Janjaweed - government-backed Arab militia - attacked on horseback and on camels (and more recently in vehicles). They came shouting racial epithets and shooting. They shot the children as they ran, they shot the elderly.

I spoke to mothers whose babies were shot from their backs, or torn from their arms and bayoneted before their eyes, whose children were tossed into bonfires. I met men whose eyes were gouged out with knives. Strong women in frail voices described their gang rapes; some were abducted and assaulted continuously over many weeks.

"No one came to help me," they said, as they showed me the brandings carved into their bodies, and tendons sliced and how they hobble now.

"Tell people what is happening here" implored one victim, Halima. Three of her five children had been killed. "Tell them we will all die. Tell them we need help." I promised her I would do my best to tell the world what is happening there. In the years since 2004, over and over and over, in camp after camp, and deep in my heart I have made this promise.

In October, I will return to the region. People will tell me their stories and again will ask for protection. I will listen, I will take more photographs, and I will keep trying to tell the world what is happening there. The people of Darfur continue to plead for protection, and still no one has come. What does this say about us?

Last week, on the Chad-Darfur border, in a region where genocide is occurring now, we lit a symbolic Olympic flame. The flame honours all those who have been lost, and those who suffer; it celebrates the courage of those who have survived, and is a symbol of hope for an end to genocide everywhere.

We lit the flame again in Rwanda where the agony of survivors is palpable - and without end. We gathered strength from their strength.

In Kigali, survivors expressed their wish to join their spirits with ours as we take the flame to other communities of survivors: Cambodia, Armenia, Germany, Bosnia.

Today, I look at Rwanda and see the abysmal failure of the United Nations and of all the nations of the world. Collectively and individually, we failed in our most essential responsibility to protect the innocent from slaughter and suffering.

We look to world leaders and our own governments and see that they are mired in self-serving interests. What are we to do about this? I tell my children that "with knowledge comes responsibility." Yet our leaders do not reflect this at all.

Most of us do not want innocent people to be slaughtered. Most of us wish others well and hope for a world in which all people everywhere can be safe. Yet, in the face of power and politics, we tend to feel overwhelmed, so we step aside and attend to our own business. The future of the world, if there is to be a future, surely lies in humility and in human responsibility. Let us draw strength and courage from the survivors of genocide and conviction from the voices of the dead.

After the Nazi Holocaust, the world vowed "never again". How obscenely disingenuous those fine words sound today. As we look at Darfur and eastern Chad - a region that has been described as "Rwanda in slow motion" - are we to conclude that "never again" applies only to white people?

I hope that caring people of the world will band together and with one voice demand an end to the terrible crime of genocide.

For more information, go to www.miafarrow.org

From Hollywood to human rights

Born to Catholic parents in 1945, Mia Farrow followed her film director father and actress mother into the industry, appearing in a number of critically acclaimed movies. Over the course of her career she has won numerous awards including seven Golden Globes. Her very public marriages and divorces to Frank Sinatra and later Woody Allen, in whose films she regularly appeared during the 1980s, meant the Farrow family were rarely out of the media spotlight.

One of Hollywood's most prolific campaigners, she has been involved in activism since the 1970s when she became an advocate of adoption rights after adopting three children from south-east Asia with her second husband André Previn. She has since gone on to adopt 11 children. A childhood survivor of the post-war polio epidemics, she has also campaigned for the eradication of the disease which has paralysed one of her adopted children. After becoming a goodwill ambassador for Unicef, she has turned her attention towards Africa and in particular, raising awareness of the genocide in Darfur.