Acts of sexual violence by armed groups "are of an unimaginable brutality that goes far beyond rape", she said.
"Women are gang raped, often in front of their families and communities. In numerous cases, male relatives are forced at gun point to rape their own daughters, mothers or sisters"
Yakin Erturk, special envoy for UN Human Rights Council on violence against women
Some of the groups were involved in the 1994 Rwandan massacres in which 800,000 people were killed.
"Women are gang-raped, often in front of their families and communities. In numerous cases, male relatives are forced at gun-point to rape their own daughters, mothers or sisters," Erturk said.
After rape, many women were shot or stabbed in the genital area, and survivors told Erturk they had been forced to eat excrement or the flesh of their murdered relatives.
Her report came after charges made by Louise Arbour, the UN high commissioner for human rights, stating that soldiers and police used excessive force and summary executions to stop protesters in the DRC earlier this year.
In the central Equateur province, the police and army often responded to civil unrest "with organised armed reprisals that target the civilian population and involve indiscriminate pillage, torture and mass rape", the report found.
Law not enforced
Although Congo's parliament outlawed sexual violence in July 2006, Erturk said the system was corrupt and in "a deplorable state", while conditions in prisons were "scandalous".
Senior army and police officers shielded their men from prosecution, and when some were arrested they escaped easily, probably "with the complicity of those in charge".
In a few cases courts had ordered the state and individuals to compensate victims.
But "to this day the government has not paid reparations to a single victim who has suffered sexual violence at the hand of state agents", Erturk said.