Fresh claims of torture and abuse at Guantanamo Bay have been published by Amnesty International to mark the US detention centre's fourth anniversary.
The London-based human rights group said 500 detainees continued to be held without charge or trial and repeated its call for the centre to be shut.
Meanwhile, cases against a Canadian and a Yemeni detainee are getting under way at tribunals at the centre in Cuba.
Such tribunals are also criticised for failing to give suspects a fair trial.
Amnesty said there were mounting allegations of appalling conditions, torture and ill-treatment of detainees at Guantanamo Bay.
'No middle ground'
The group highlighted the case of Jumah al-Dossari - a 32-year-old Bahraini who was taken to Guantanamo Bay in January 2002.
His lawyer said Mr Dossari had been urinated on, threatened with rape and had his head repeatedly smashed against the floor. The claims have been denied by the US government.
He had reportedly attempted to commit suicide 10 times, the rights group said.
Another detainee, Sami al-Hajj - a Sudanese cameraman for Arab satellite TV network al-Jazeera - had been subjected to severe physical, sexual and religious abuse over the last four years, his British lawyer Clive Stafford-Smith said.
Amnesty's UK Campaigns Director, Stephen Bowen, called the situation at Guantanamo Bay "shocking".
"There's no middle ground regarding Guantanamo. It must be closed and there must be an investigation into the dozens of torture reports that have emerged since 2002," he said.
Supreme Court ruling
On Wednesday, Ali Hamza al-Bahlul, a Yemeni accused of being Osama Bin Laden's bodyguard, was appearing before a military tribunal for a pre-trial hearing.
He has asked to defend himself against a charge of conspiracy to commit war crimes.
A pre-trial hearing is also being held for 19-year-old Canadian detainee Omar Khadr, who is accused of killing a US army medic in a firefight at a suspect al-Qaeda base in Afghanistan.
If convicted, the two men could face life in prison.
The cases have gone ahead even though the trials of other Guantanamo detainees have been halted while the US Supreme Court decides if US President George W Bush had the authority to establish such tribunals.
The Bush administration argues that it needs flexibility in dealing with the war on terror and terrorists cannot be treated as if they are just another criminal defendant.
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