BAGHDAD — The torture of Iraqi detainees at a secret prison in Baghdad was far more systematic and brutal than initially reported, Human Rights Watch reported on Tuesday.

The existence of the prison, which housed mostly Sunni Arab prisoners, has created a political furor in Iraq, prompted government denials and fanned sectarian tensions.

“Abu Ghraib was a picnic” compared with the secret prison, said Sheik Abdullah Humedi Ajeel al-Yawar, one of the most influential Sunni Arab tribal leaders in the northern province of Nineveh, where the detainees were rounded up by Iraqi soldiers based on suspicions that they had links to the insurgency and brought to Baghdad with little due process. Abu Ghraib is the prison at which American guards tortured Iraqi prisoners, severely damaging Iraqis’ trust in the United States.

Human Rights Watch gained access on Monday to about 300 male detainees transferred from the once secret prison at the Old Muthanna military airfield to the Rusafa prison in Baghdad and documented its findings, which it described as “credible and consistent,” in a draft report provided to The New York Times on Tuesday by the rights group.