It accused the US of violating detainees' rights to a fair trial, to freedom of religion and to health. It recommended that the US close the prison camp in Cuba and revoke all special interrogation techniques authorised by the US defence department.
The draft report, from five UN human rights experts, says: "The apparent attempts by the US administration to reinterpret certain interrogation techniques as not reaching the threshold of torture in the framework of the struggle against terrorism are of utmost concern."
American officials rejected the report, saying it was riddled with errors and treated statements from detainees' lawyers as fact. Its most significant flaw, officials said, was that it judged the treatment of detainees according to peacetime human rights laws. The US contends that it is in a state of conflict and should be judged according to the laws of war.
"Once you fail to even acknowledge that as the legal basis for what we're doing, much of the legal analysis that follows just doesn't hold," a US state department official said.
The US is holding about 500 people at Guantanamo on suspicion of links to al-Qaeda or the old Taleban regime in Afghanistan. It has filed charges against only ten of the detainees.
The draft report, which will be presented to the 53-nation UN Human Rights Commission, dismissed the US claim that the war on terror constituted an armed conflict. It also said the prisoners had a right to challenge their detention, and that that right was being violated. "In the case of the Guantanamo Bay detainees, the US executive operates as judge, as prosecutor, and as defence counsel," the report said.
The five experts had sought invitations from the US to visit the camp since 2002. They turned down an invite last year because they were told they would not be allowed to interview detainees.