Iraq's main Sunni political party has called for an international inquiry into the alleged abuse of more than 170 detainees by Iraqi forces in Baghdad.
Sunni groups have released images of alleged torture by police
Iraqi Prime Minister Ibrahim Jaafari has ordered an investigation.
But the Islamic Party said Iraqi-led investigations into past cases of abuse had not produced results.
Washington is backing the Iraqi inquiry - but the US is itself facing pressure to be more transparent about the treatment of its prisoners.
The Iraqi abuse allegations came to light when prisoners, many malnourished and some showing signs of apparent torture, were found by US troops on Sunday.
An Iraqi Islamic Party spokesman said only an international inquiry would get to the bottom of the alleged abuse.
"There have been similar cases in the past and investigations into them led to nothing," Iyad al-Samarrai, the party's spokesman, told AFP news agency.
"We want an international and impartial inquiry as we are beginning to think there are people high up in government who are responsible, or at least accomplices."
Another party spokesman, Alaa Makki, accused US-led forces of giving the alleged abuses "the green light".
Washington has said it is troubled by the alleged abuse, and that it neither practices torture nor believes others should.
Nevertheless, the discovery comes as fresh allegations surface of US troops mistreating detainees in Iraq.
Two former Iraqi prisoners have told US TV they were beaten, fired at with rubber bullets and subjected to mock executions at the hands of US troops in 2003.
The US has also lobbied against legislation that would ban all inhumane treatment of detainees, and is under increasing international pressure to answer allegations that the CIA is operating secret prisons abroad.
The 173 Iraqi prisoners were found in a US raid on an Iraqi interior ministry building in the central Jadiriya district of Baghdad.
It followed repeated enquiries by the parents of a missing Iraqi teenager.
Most of those held were Sunni Arabs.
Iraq's prime minister has promised to find those responsible for any abuse and to put all his weight behind the inquiry.
The allegations are a deep embarrassment for the Iraqi government, but, however shocking, they will not come as a major surprise to many Iraqis, the BBC's Caroline Hawley in Baghdad says.
There have been persistent allegations of abuse by members of the Shia-dominated security forces, she says.
But Sunday's discovery is hard evidence and officials believe it may be the tip of the iceberg.
Iraqi security forces have faced repeated allegations of abuse
There are suspicions the building may also have been used as a base for a militia called the Badr Brigade, our correspondent adds.
Deputy interior minister Hussein Kamal, who saw some of the abuse victims, said: "I saw signs of physical abuse by brutal beating, one or two detainees were paralysed and some had their skin peeled off various parts of their bodies."
Dr Laith Kubba, a spokesman for Mr Jaafari, said methods used under Saddam Hussein had not been completely eradicated despite efforts to introduce new practices.
The security forces have faced repeated allegations of systematic abuse and torture of detainees, and of extra-judicial killings.
A report by pressure group Human Rights Watch earlier this year said methods used by Iraqi police included beating detainees with cables, hanging them from their wrists for long periods and giving electric shocks to sensitive parts of the body.