Human Rights Watch said its researchers interviewed 14 former detainees, some of whom belonged to anti-Gadhafi militias, who said they were tortured while in U.S. custody during the administration of George W. Bush.
The organization, in its 154-page report, said some former detainees now hold political office in post-war Libya.
"One former detainee alleged he was waterboarded and another described a similar form of water torture, contradicting claims by Bush administration officials that only three men in U.S. custody had been waterboarded," the rights organization said in a statement.
In April, Leigh Day and Co., a law firm representing Libyan military chief Abdul Hakim Belhadj, claimed former British Foreign Secretary Jack Straw was complicit in the rendition of the Libyan commander.
Belhadj, Libya's military commander who worked with NATO forces during Operation Unified Protector last year, claimed London played a role in his illegal rendition and torture while he was held in Libya's Abu Salim jail before the war.
"The involvement of many countries in the abuse of Gadhafi's enemies suggests that the tentacles of the U.S. detention and interrogation program reached far beyond what was previously known," said Laura Pitter, counter-terrorism adviser at Human Rights Watch and author of the report, in a statement.