Bad timing for al-Maliki
The discovery of the prison comes at a sensitive time for al-Maliki as he tries to negotiate alliances with other parties, following an inconclusive election in March.
HRW interviewed 42 of the 300 men who were detained on a military base at Baghdad's old Muthanna airfield after being arrested in Mosul and accused of terrorism.
"The men's stories were credible and consistent. Most of the 300 displayed fresh scars and injuries they said were a result of routine and systematic torture they had experienced at the hands of interrogators at Muthanna," HRW said.
The detainees said they were handcuffed, blindfolded and hung upside down, and that interrogators kicked, whipped and beat them.
Interrogators also placed dirty plastic bags over their heads to close off air supply. When the detainees passed out, they were awoken with electric shocks to the genitals or other parts of the body, HRW said.
One detainee, a former Iraqi army general who had been living in London but returned to Mosul after his son was detained, said his jailors refused to give him medicine for his diabetes and high blood pressure and beat him severely.
"They applied electricity to my penis and sodomized me with a stick," the man, who is in a wheelchair, told Human Rights Watch. "I was forced to sign a confession that they wouldn't let me read."
Another detainee, who was 21, said interrogators threatened to rape his mother and sisters if he did not confess. During one torture session, guards made another detainee rape him.
Another detainee said he was sodomised with a pistol.
The Human Rights Ministry says three Iraqi army officers have been arrested for questioning. The prison was illegal because it was not under the jurisdiction of the Justice Ministry and the Human Rights Ministry was not informed of its existence.
"What happened at Muthanna is an example of the horrendous abuse Iraqi leaders say they want to leave behind," said Joe Stork, deputy Middle East director at Human Rights Watch.
"Everyone responsible, from the top on down, needs to be held accountable."