"The use of evidence tainted by torture and other inhuman treatment is pervasive and systematic in the cases of prisoners held at Guantanamo Bay, and has already infected legal judgments made there," Human Rights First said in a report titled "Tortured Justice."
By hearing testimony extracted under torture when trying Guantanamo detainees, the United States is "tainting the legitimacy of the proceedings, both at home and in the eyes of the international community; alienating US allies and empowering terrorists," it said.
A Pentagon spokesman defended the legal process set up to try "war on terror" detainees.
The Human Rights First report "makes ill-founded assumptions about the evidence upon which our military prosecutors will rely," said Commander Jeffrey Gordon.
"In fact, military prosecutors are building cases based on solid evidence that will withstand the scrutiny under both the law and in the eyes of the public," he said.
The report was released two days after President George W. Bush vetoed legislation on intelligence funding because it called for interrogation methods to be limited to techniques outlined in a US military manual.
That would have excluded waterboarding, a method of simulated drowning widely seen as a form of torture.
In vetoing the bill on Saturday, Bush called hardcore interrogation methods "one of the most valuable tools in the war on terror," and praised them for preventing another attack on the United States, such as those on September 11, 2001.
But according to scientific studies and former interrogators cited in Monday's report, torture and other forms of cruelty do not produce useful evidence.
"There is no scientific record that establishes that torture and coercion are going to produce accurate or reliable information," said Avi Cover, co-author of the report.
"Tortured Justice" quoted former FBI interrogator Jack Cloonan as saying the abusive interrogations conducted at the US detention facility at Guantanamo were "a complete and unmitigated failure."
Better results were obtained by providing suspects with legal representation and giving them a fair hearing.
"A tremendous amount of information came our way as a result of treating people humanely," Cloonan was quoted as saying.
The military commissions at Guantanamo were created by an act of Congress in 2006, which also allowed coerced evidence to be used against some suspects.
"This system flies in the face of law that has been in place for more than 200 years in this country and taints the legitimacy of the proceedings at home and internationally," said HRF's Debra Colson, one of the lead authors of the report.
Allowing evidence obtained by torture would be "inconsistent with the justice system of a civilized society", said Cover.
The report cites the cases of six Guantanamo detainees who were allegedly abused by US government interrogators, but also covers interrogations that took place at secret rendition prisons operated by the United States in Afghanistan, Iraq, "all over," said Cover.
"The techniques go well beyond waterboarding. Stress and duress techniques were quite common and alleged by many people. Beatings, isolation, sensory deprivation were alleged by many," he said.
"These tactics were part of the catalogue of interrogation techniques used by communist countries during the Cold War that were studied and many times adopted by the CIA," Cover said.
"While it was found often that they produced statements, they have no real record of producing reliable information."
Torture and other inhuman treatment are banned under US criminal, military and international law, the report said.
"The Supreme Court has repeatedly held that due process prohibits the government's use of involuntary statements extracted through psychological pressure, physical intimidation, torture or other mistreatment," it said.
"As the military commission proceedings gather momentum in 2008, the American public and the international community will be watching," it added.
© 2008 Agence France Presse