London (AFP) ― The family and supporters of “one of the most abused prisoners in Guantanamo” on Tuesday launched a new celebrity-backed campaign demanding his release, coinciding with the publication of his prison diary.

Oscar winner Colin Firth and fellow actors Jude Law, Benedict Cumberbatch and John Hurt are among those who will read out extracts from Mohamedou Ould Slahi’s “Guantanamo Diary” to appear on the Guardian newspaper’s website.

U.S. district court judge James Robertson ordered that Slahi be released in 2010 due to lack of evidence that he was directly involved in al-Qaida terror plots, but he remains in detention after the Department of Justice appealed the decision.

Human rights activist Larry Siems, the book’s editor, Slahi’s lawyer Nancy Hollander and brother Yahdih described the battle to release the memoirs and his current legal limbo during a press conference in London.

“It’s a tragic situation,” said Hollander. “He’s never been charged with any crime.”

Siems, who described Slahi as “one of the most abused prisoners” in the military prison explained that it took a seven-year legal battle before U.S. authorities finally cleared the manuscript, but only after demanding 2,500 changes.

Slahi said he only confessed to involvement in various terror plots, including one to bomb the CN Tower in Toronto, after being tortured and humiliated.

The 44-year-old swore allegiance to al-Qaida after travelling to Afghanistan in the 1990s and fought against the Soviet-backed regime there, but claims he left the group in 1992.

He was detained in his home country of Mauritania following the Sept. 11 attacks on the United States in 2001, on suspicion of involvement in an unsuccessful plot to bomb Los Angeles in 1999, and was taken to Guantanamo in 2002 following interrogation in Jordan and Afghanistan.

In the book, Slahi said he was beaten during a three-hour boat journey in the Caribbean, made to drink salt water and subjected to death threats and sleep deprivation.

The American Civil Liberties Union has already launched an online petition demanding his release.

“The U.S. government’s justifications for holding Slahi fail because he has never taken part in any hostilities against the United States. And he poses no threat to the United States,” says the website.

“While at Guantanamo Bay prison, Slahi was the victim of one of the military’s most brutal torture regimens, approved by former Defense Secretary Donald Rumsfeld himself,” it adds.

Roxy Music member Brian Eno will perform a live reading of a passage from the book at a London event organized by the Guardian to be held later Tuesday.

Other celebrities to back the campaign with readings include musician Nick Cave, author Philip Pullman and comedian Stephen Fry.