LONDON, Oct 25 (Reuters) - The WikiLeaks website began publishing on Thursday what it said were more than 100 U.S. Defense Department files detailing military detention policies in camps in Iraq and at Guantanamo Bay in the years after the Sept. 11 attacks on U.S. targets.

In a statement, WikiLeaks criticised regulations it said had led to abuse and impunity and urged human rights activists to use the documents to research what it called "policies of unaccountability".

The statement quoted WikiLeaks founder Julian Assange as saying: "The 'Detainee Policies' show the anatomy of the beast that is post-9/11 detention, the carving out of a dark space where law and rights do not apply, where persons can be detained without a trace at the convenience of the U.S. Department of Defense."

"It shows the excesses of the early days of war against an unknown 'enemy' and how these policies matured and evolved, ultimately deriving into the permanent state of exception that the United States now finds itself in, a decade later."

A spokeswoman for the U.S. Embassy in London said they had no immediate comment.

In January, U.N. human rights chief Navi Pillay said the United States was still flouting international law at Guantanamo Bay by arbitrarily and indefinitely detaining individuals.

Almost 3,000 people were killed in 2001 when militants from Osama bin Laden's al Qaeda flew hijacked airliners into the World Trade Center towers in New York, the Pentagon and a field in Pennsylvania.

Then President George W. Bush set up a detention camp at a U.S. naval base at Guantanamo in Cuba after U.S.-led forces invaded Afghanistan to expel al Qaeda following the Sept. 11 raids. Of the 779 men held there, 167 remained as of mid-September 2012.

WikiLeaks said a number of documents it was releasing related to interrogation of detainees, and these showed direct physical violence was prohibited.

But it added the documents showed "a formal policy of terrorising detainees during interrogations, combined with a policy of destroying interrogation recordings, has led to abuse and impunity".

A number of what can only be described as "policies of unaccountability" would also be released, it said.

One such document was a 2005 document "Policy on Assigning Detainee Internment Serial Numbers", it said.

"This document is concerned with discreetly 'disappearing' detainees into the custody of other U.S. government agencies while keeping their names out of U.S. military central records - by systematically holding off from assigning a prisoner record number," the WikiLeaks statement said.

WikiLeaks did not elaborate. But human rights activists say that after the Sept. 11 attacks, the Central Intelligence Agency used "black sites" in friendly countries to interrogate and sometimes torture suspected militants beyond the reach of normal legal protections. The programme's existence has never been officially acknowledged.

Assange, whose website previously angered the United States by releasing thousands of U.S. diplomatic cables, has been holed up inside Ecuador's embassy in central London since June to avoid extradition to Sweden to face rape and sexual assault allegations. He denies wrongdoing.