An international human rights group is warning that the U.S. government's investigation of the Sept. 11 attacks has been marred by arbitrary detentions, due process violations and secret arrests.

In a 95-page report, Presumption of Guilt: Human Rights Abuses of Post-September 11 Detainees, Human Rights Watch concludes the U.S. Department of Justice has misused immigration charges to dodge legal restraints on its power to detain and interrogate people as it pursues its terrorist probe.

"An immigration violation should not give the government license to rip up the rule book," said Jamie Fellner, director of Human Rights Watch's U.S. Program. "By restricting judicial oversight and blocking public scrutiny, the government has exercised virtually unchecked power over those it has detained."

The report, based on interviews with current and former detainees and their attorneys, found the U.S. government has held some detainees for prolonged periods without charges, impeded their access to counsel, subjected them to coercive interrogations and overridden judicial orders to release them on bond during immigration proceedings. In some cases, the government has incarcerated detainees for months under restrictive conditions, including solitary confinement. Some detainees, it is alleged, were physically and verbally abused because of their national origin or religion.

An estimated 1,200 non-citizens have been secretly arrested and incarcerated in connection with the Sept. 11 investigation. The vast majority are from Middle Eastern, South Asian, and North African countries.

The report describes cases in which random encounters with law enforcement or neighbours' suspicions based on no more than national origin and religion led to interrogation about possible links to terrorism.

More than 700 individuals were held on immigration charges while the government continued to investigate them. Human Rights Watch says the Department of Justice kept them in detention until it decided they had no links to or knowledge of terrorism. None has been indicted for terrorist-related crimes.

Fellner says using immigration law violations to detain these individuals while they were criminally investigated enabled the Justice Department to deny non-citizens their rights under U.S. criminal law, such as the right to court-appointed counsel and the right to be promptly charged after arrest. In some cases, he claims, the Justice Department flouted regular procedures to keep non-citizens in the custody of the Immigration and Naturalization Service on the off chance they might be found to be engaged in terrorism, a practice that amounts to unlawful "preventive detention."

"The U.S. government has failed to uphold the very values President Bush declared were under attack on Sept.11," Fellner said. "It has ignored basic restraints on a government's power to detain that are the hallmarks of free and democratic nations."

The report also criticizes the U.S. government for blocking the public's right to know by conducting secret arrests and secret hearings.