Thousands of women and girls were subjected to individual and gang rape, according to a report issued by Human Rights Watch on Thursday.
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Child combatants raped women who were old enough to be their grandmothers
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Human Rights Watch
The 75-page document describes alleged abuses committed by the rebel Revolutionary United Front (RUF), as well as other rebel, government and international peacekeeping forces.
The report - entitled We'll Kill You If You Cry - is based on hundreds of interviews with victims, witnesses and officials.
"We have documented unimaginable atrocities against women in Sierra Leone," said Human Rights Watch's Peter Takirambudde.
The report says most of the sexual violence was committed primarily by soldiers of various rebel forces - the RUF, the Armed Forces Revolutionary Council (AFRC), and the West Side Boys.
The document also examines crimes by government forces, and gives details of alleged rapes by foreign peacekeepers - including soldiers from Guinea, Nigeria, Ukraine, and Bangladesh.
Atrocities
Sexual violence was characterised by extraordinary brutality, and frequently accompanied by other abuses against the victim, her family and her community, according to the report.
"Child combatants raped women who were old enough to be their grandmothers, rebels raped pregnant and breastfeeding mothers, and fathers were forced to watch their daughters being raped," it says.
Mr Takirambudde said: "The war in Sierra Leone became infamous for the amputation of hands and arms.
"Rape may not be visible in the same way, but it is every bit as devastating."
About 30,000 people were killed - and many thousands were maimed - during the conflict between 1991 and 2001.
The United Nations has set up a Special Court for Sierra Leone to try alleged human rights violations committed by all parties during the war.