They threatened to whip Fatima if she didn't pay a fine, exploiting her forced pregnancy to turn her into the victim of an extortion racket.

Hawa [left] with her two-month-old son, and Fatima
"They asked me who was the father of my baby," Fatima remembered, twisting a piece of paper between her fingers. "I told them that I didn't know. There were seven men on horses. Three of them raped me and four of them beat my mother. We had gone to get onions from our farm."

The police threw Fatima in prison in Bendisi, western Sudan, while her family tried to scrape together the 20,000 dinar (£45) fine - a large sum in a country where the average annual wage is just £200.

She spent three days with no food and nothing to sleep on but the bare earth before her father managed to collect 12,000 dinars from relatives and pay it last week. He is still trying to find the rest.

"The police said he must pay it by the time my baby is delivered or I will be whipped," Fatima said.

Thousands of women and girls have been raped since 2003, when local tribes in Darfur took up arms in protest at what they perceived to be neglect and discrimination by Sudan's government in Khartoum.

Many of the rape victims have been branded to ensure that they never escape the stigma. Most identify their attackers simply as "Janjaweed", a generic term for the nomadic, Arabic-speaking gunmen who often work in concert with the Sudanese armed forces.

Médecins Sans Frontières, the aid agency, has treated nearly 400 rape victims in Darfur in the past six months but believes that the number of victims is far higher. Many are too ashamed or frightened to report what has happened.

The women, many of whom are on their own - their houses having been destroyed and their relatives killed - are charged with having extramarital sex even though under Islamic law, a woman who is raped is not considered to be guilty of a crime.

"Rape is being used as a deliberate way to fragment the family and community," said one aid worker, speaking on condition of anonymity. "Many of these women are raped by soldiers and police as well [as the Janjaweed]."

In Bendisi, one 18-year-old girl is huddled in a corner as she remembers the police threats. "They told me that I could pay 15,000 dinars or I could be raped 40 times," she said. "They will take my money now but they never heard my cry when the Janjaweed came for me."

Other women claimed that police officers had raped them while in prison, promising shorter sentences or smaller fines.

Sixteen-year-old Hawa, who was gang-raped by three men while collecting firewood, cradles her two-month-old son Hamoudi in a ragged green blanket. Her shawl has several holes in it and it is her only possession. She is sleeping rough after her grandmother threw her out because she was pregnant.

"The police held me for 10 days in a cell. They didn't give me any food and there was nowhere to sleep," she whispered, tucking the blanket around the face of her sleeping son.

"I told them I have no money. They whipped me on my chest and my back. I was bleeding a lot." She was eight months pregnant, and terrified.

She has been freed from prison but the police are still demanding 20,000 dinars.

Some officials at least are aware of the police practice. When a judge visited from Garsila, a nearby town where similar cases have been reported, he allegedly warned officers to stop recording women's names in case the list should be used as evidence against them.

The arrests, the fines and the whippings continue, however. "Who will want to marry me now?" asked Fatima, her eyes filling with tears. "Maybe an old man, more than 50. I am destroyed. I have lost my chance in life."

Last week Kofi Annan, the Secretary-General of the United Nations, told the Security Council that action had to be taken to end "appalling" crimes in western Darfur.

"We keep getting reports which show that the killing and raping and burning is still going on," he said.

Mr Annan urged the 15-member council to pass a resolution imposing sanctions on those who block peace moves in Darfur. More than 200,000 people are believed to have lost their lives in the two-year conflict.