A 54-page report published on Thursday detailed the findings of five UN experts who had sought to interview detainees at the detention centre but were refused by American authorities.
It recommended the US "close down the Guantanamo Bay detention centre and to refrain from any practice amounting to torture or cruel, inhuman or degrading treatment."
A spokesman for the White House, Scott McClellan dismissed the report as "a rehash" of previous allegations made by lawyers acting on behalf of some Guantanamo detainees.
"These are dangerous terrorists that we're talking about that are there," he said, "we know that al-Qaida terrorists are trained in trying to disseminate false allegations."
McClellan said the UN makes many serious investigations of human rights abuse but "this was not one of them."
He said the military treats all the prisoners humanely.
Need for closure
Currently about about 490 men are being held at the Guantanamo Bay camp on the southeastern tip of Cuba.
"These are dangerous terrorists that we're talking about"
Scott McClellan, White House spokesman
The UN report was backed by Secretary General Kofi Annan, who said he believed the US should close the prison as soon as possible.
While he said he didn't necessarily agree with everything in the report, Annan said he did support its opposition to people being held without being charged or given a chance to plead their case in court.
"I think sooner or later there will be a need to close Guantanamo, and I think it will be up to the government to decide, and hopefully to do it as soon as is possible," Annan said.
The US ambassador to the UN in Geneva, Kevin Moley, claimed that the investigation had taken little account of evidence provided by the US, and that the five UN experts rejected an invitation to visit Guantanamo.
In a response that was included at the end of the report, Moley said: "It is particularly unfortunate that the special rapporteurs rejected the invitation and that their unedited report does not reflect the direct, personal knowledge that this visit would have provided."
Conditions inside the prison are
criticised by the UN
A preliminary version of the report was leaked earlier this week before it included the US comment.
The five UN experts who authored the report had sought invitations from the US to visit Guantanamo Bay since 2002.
Three were invited last year, but refused in November after being told they could not interview detainees.
Only the International Committee of the Red Cross has been allowed to visit detainees at the camp, but the organisation keeps its findings confidential, reporting them solely to the detaining power.
The UN report's findings, which were being made public, were based on interviews with former detainees, public documents, media reports, lawyers and a questionnaire filled out by the US government.
Many of the allegations have been made before, but the document represented the first inquiry launched by the 53-nation UN Human Rights Commission, the global body's top rights watchdog.
The group of UN investigators included Leila Zerrougui, an expert on arbitrary detention; Leandro Despouy, expert on judicial independence; Manfred Nowak, expert on torture; Asma Jahangir, an expert on freedom of religion; and Paul Hunt, expert on physical and mental health.
The five were appointed by the commission to the three-year project. They worked independently, with expenses covered but received no payment from the UN.
The five come from Argentina, Austria, New Zealand, Algeria and Pakistan.
The US, which is a member of the commission, has criticised the body itself for including members from countries with poor human rights records.
The Guantanamo detainees are accused of having links to Afghanistan's ousted Taliban government or al-Qaida, though only a handful have been charged since the camp opened in January 2001.
Although the investigators did not visit Guantanamo, they said photographic evidence - corroborated by testimony of former prisoners - showed that detainees were shackled, chained, hooded and forced to wear earphones and goggles.
They said prisoners were beaten, stripped and force shaved if they resisted.
The report said: "Such treatment amounts to torture, as it inflicts severe pain or suffering on the victims for the purpose of intimidation and/or punishment."
Some of the interrogation techniques used at the detention facility itself - particularly the use of dogs, exposure to extreme temperatures, sleep deprivation for several consecutive days and prolonged isolation - caused extreme suffering, the report said.