The report by five human rights experts from the UN accused the US of violating the detainees' rights to a fair trial, to freedom of religion and to health.
It recommended that the US close the detention centre and revoke all special interrogation techniques authorised by the US Department of Defence.
The draft report said: "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."
US 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 US treatment of detainees according to peace-time human rights laws.
The US contends that it is in a state of conflict and should be judged according to the laws of war.
An official from the US State Department said: "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."
"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"
Draft UN report
The official spoke on condition of anonymity because the US has not formulated an official public response.
The draft report was delivered to the US on 16 January. It was first disclosed late on Sunday by the Los Angeles Times, and a copy was obtained by journalists.
The five experts have mandates that cover torture, freedom of religion, health, independent judiciary and arbitrary detention. They started working together in June 2004 to monitor conditions at Guantanamo Bay.
Important UN group
They were appointed to their three-year terms by the 53-nation UN Human Rights Commission, the global body's rights watchdog.
The US holds about 500 people on suspicion of links to al-Qaida or Afghanistan's ousted Taliban government at the base in Guantanamo. It has filed charges against 10 detainees.
The draft report, which will be presented to the next session of the commission, dismissed the US claim that the "war on terror" constitutes an armed conflict.
It also said the detainees at Guantanamo Bay had a right to challenge their detention, and that right was being violated.
The report said: "In the case of the Guantanamo Bay detainees, the US executive operates as judge, as prosecutor, and as defence counsel. This constitutes serious violations of various guarantees of the right to a fair trial."
The five experts had sought invitations from the United States to visit Guantanamo Bay since 2002. When it was offered last year, they turned it down because they were told they would not be allowed to interview detainees.
The UN appointed a human rights
team to look into Guantanamo
On Monday, US officials faulted the experts for rejecting the invitation, saying it undermined the accuracy of their findings.
Meanwhile, lawyers for two Chinese Muslims held at Guantanamo Bay want a federal appeals court to order their release inside the US.
In court papers filed on Monday, lawyers for Abu Bakker Qassim and A'Del Abdu al-Hakim, captured in Pakistan in 2001, said they should be released because the US military found that they are not "enemy combatants".
The lawyers want the appellate court to overturn a ruling by U.S. District Judge James Robertson, who ruled in December that the men are being detained illegally but said he could not force the U.S. government to release them.
The two men, along with several other Uighurs, have been held at Guantanamo Bay since 2002 because the US government has been unable to find a country willing to accept them. US officials cannot send them back to China because of the possibility they would be tortured or killed.