US-based Richard Bourke, who has been working for almost two years on behalf of dozens of detainees at Guantanamo Bay in Cuba, said the methods "clearly" fell under the definition of torture under international conventions.
"They are engaging in good old-fashioned torture, as people would have understood it in the Dark Ages," he told an Australian radio station.
About 660 prisoners captured in Afghanistan and elsewhere are being held at Guantanamo, without charges or access to lawyers - some since January 2002.
The US government rarely comments on activities at the prison which has been dubbed Camp X-Ray because of the see-through chicken-wire fencing in which inmates were originally held.
Bourke said his claims were based on reports leaked by US military personnel and from descriptions by some detainees that had been released.
"...they were being made to kneel cruciform in the sun until they collapsed"
"One of the detainees had described being taken out and tied to a post and having rubber bullets fired at them. They were being made to kneel cruciform in the sun until they collapsed," he said.
Media reports that many detainees have attempted suicide and are suffering mental health problems, backing up claims of harsh treatment, he added.
US officials deny charges
Earlier this year, US officials denied using torture and said detainees were interrogated humanely, allowed to practice their religion and given good medical care.
However, families are denied access and can only communicate with detainees through heavily censored mail. Human rights groups and the media have been given only limited and strictly controlled access.
Civil liberties violated
The International Committee of the Red Cross (ICRC) says the detainees are suffering growing psychological problems with their continuing incarceration.
ICRC spokeswoman Amanda Williamson said her organisation had witnessed a “worrying” deterioration of the psychological well-being of the detainees.
The American Civil Liberties Union and four other groups, including Physicians for Human Rights and Veterans for Peace, are seeking records pertaining to the treatment of detainees held in US custody as well as to those who have been handed over to other countries for interrogation, the Washington Post reports.
Use of coercive interrogation techniques would constitute clear violations of both domestic and international law, the groups say.
The groups plan to file a formal request with the US government to gain information on the status of the prisoners.
Civil liberties groups want more
information on prisoners situation
Last December, the paper cited former and current national security officials reporting that detainees in Afghanistan and elsewhere were routinely subjected to "stress and duress" techniques, including being held in awkward positions or being deprived of sleep with a 24-hour bombardment of lights.
The Washington Post reported captives had been handed over to foreign intelligence services known for using questionable interrogation tactics - including Jordan, Egypt and Morocco.
Prisoners released from Guantanamo and Bagram air base in Afghanistan have said in interviews with Amnesty International that they are subjected to human rights abuses, including sleep deprivation and forced injections of drugs.
Two deaths at Bagram in December 2002 are also under investigation after being classified as homicides by military pathologists.
Appeal to governments
Bourke has said governments around the world must stand up to the US government and demand that the United Nations should investigate the reports of torture.
Almost all the detainees, from more than 40 countries, are said to be members of al-Qaida network or the ousted Afghan Taliban.
They are to be tried by secret military tribunals. The US government says they could be held until it declares an end to its so-called "war on terrorism."