Human rights workers and 20 former inmates at Guantanamo Bay and other U.S. terror-suspect prisons abroad convened a conference here Friday to bring new pressure on Washington to end what they called systematic torture and unjustified detention.

"Torture should have been kept where it belonged, in the 16th century, instead of being imported into the 21st," said Irene Khan, secretary general of Amnesty International, which is co-hosting the meeting. Ten other foreign prisoners will testify by videotape, in what organizers call the largest gathering ever of former Guantanamo prisoners and prisoners' families.

The conference opens as the Bush administration is facing increasing criticism abroad over its open-ended detention of terror suspects. U.S. officials contend that the unique threat that international terror groups pose to the United States justifies continued operation of the prisons. Inmates are treated humanely, officials have said, and when reports of mistreatment surface, prompt legal action is taken against the perpetrators.

The White House has also been criticized because of a report in The Washington Post that the CIA has been interrogating top al Qaeda captives in a secret prisons in countries that at various times have included Thailand, Afghanistan and several in Eastern Europe.

At a news conference, Khan called for the European Union and the United Nations to launch an investigation into which countries are hosting the secret CIA prisons. "It's about time that this veil of secrecy is broken," she said.

On Friday, U.N. investigators formally canceled a scheduled visit to Guantanamo, citing U.S. officials' refusal to allow them to speak privately with detainees at the facility in Cuba. Manfred Nowak, the U.N. special rapporteur on torture, said here that speaking with detainees alone was the "minimum standard of objective fact-finding" and that U.N. officials could not conduct a meaningful assessment without doing so.

"The writ of international human rights does not stop at the gates of Guantanamo Bay," said Paul Hunt, a U.N. official who was denied U.S. permission to join the planned mission to Guantanamo.

A Department of Defense spokesman said by e-mail Friday that "the level of access that DOD provides to its detention facilities is unprecedented in a time of war." The department has provided access to the International Committee of the Red Cross, members of Congress and the news media and extended an invitation to the U.N. Special Rapporteurs, the spokesman noted.

This invitation, the spokesman wrote, was extended "in an effort to broaden understanding of U.S. detention operations and to demonstrate that detainees at Guantanamo are treated humanely."

One of the former prisoners at the conference is Moazzam Begg, 37, a British citizen who was arrested by U.S. forces at his home in Islamabad, Pakistan, in January 2002. By his account, he was held in Kandahar and Bagram, Afghanistan, as well as Guantanamo, before his release last January. Begg, who was never charged with a crime, said he was subjected to and witnessed "things that we would believe are out of a Nazi manual," including prisoners being sodomized with sticks.

In an interview and in a report prepared by his lawyers, Begg said that during his time at Bagram, U.S. soldiers beat him with fists, kicked him, forced him to take cold showers in freezing winter temperatures, placed guns to his head and threatened to harm his 6-year-old daughter. Begg also said he witnessed beatings by U.S. soldiers that resulted in the deaths of two detainees at Bagram. He said one was a young Afghan who escaped from a "cage" where he was being held. Begg said he heard two American soldiers severely beating the Afghan. He said he then saw the soldiers carry the man to a medical station, and his dead body was taken out on a gurney a short while later.

The second case involved an Afghan man who was forced to stand with his hands cuffed to a bar high over his head, Begg said. By his account, the man eventually went limp from the pain, and a guard ordered him to stand up straight. When he did not, the guard beat him "mercilessly" for "a long time," Begg said, adding that the man later died.

There has been no independent corroboration of Begg's accusations. He said that FBI investigators recently interviewed him by telephone about the first case. He was questioned by U.S. military investigators at Guantanamo about the second case, he said.

The U.S. military has charged 14 soldiers at Bagram in the abuse of inmates and the deaths of two prisoners. It was unclear whether the deaths Begg said he had seen were the same as those deaths. So far, six soldiers have been convicted of or pleaded guilty to abuse charges and three have been acquitted.

Researcher Robert Thomason in Washington contributed to this report.

Moazzam Begg, who said he was held in U.S. prisons in Afghanistan and Cuba but never charged, reacts during conference in London.