UN High Commissioner for Human Rights Navi Pillay says her office has documented a "broad pattern of torture and ill-treatment" in government facilities and some run by rebel groups.
Pillay said in a statement today that the report draws on detailed testimony from victims and witnesses.
The abuses include rape, beatings and burnings, sometimes leading to death.
Pillay's office has documented widespread abuses by the government and rebel groups in Syria's three-year civil war, prompting calls for war crimes inquiries.