The Iraqi Government was facing demands for an international inquiry after the discovery of a secret prison in Baghdad where around 170 detainees were reportedly tortured, beaten, burnt and starved.

Sunni Arab leaders have seized upon the chance discovery of the detention facility in the basement of an Interior Ministry complex as evidence that the Shia-led coalition is using torture techniques reminiscent of Saddam Hussein's regime.

"We insist on having an international investigation," said Alaa Makki, a spokesman for the Sunni-dominated Islamic Party.

"There have been similar cases in the past and investigations into them led to nothing. 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."

Reports suggest that troops of the Third Infantry Division stumbled on the prison on Sunday night while searching for a missing 15-year-old boy.

The troops forced their way past guards at the centre in Jadriya, a suburb a mile south of the Green Zone, and discovered scores of men and teenage boys, many malnourished and showing signs of having been tortured. An Interior Ministry statement said "instruments of torture," which it did not describe, were found in the building.

Ibrahim Jaafari, the Prime Minister, ordered an immediate internal investigation into the allegations, with a committee due to report within two weeks. The Prime Minister said that he had appointed a deputy prime minister, Rowsh Shaways, who is Kurdish, to head the inquiry.

"We want to know how this was allowed to happen, and how things reached this point," Mr Jaafari said. There will also be a wider investigation into jail conditions across the country be led by "ministers and other figures," he said.

Further details of conditions within the tiled bunker - used as a bomb shelter during Saddam's rule - emerged today. The Reuters news agency quoted an Iraqi guard who took part in the detentions saying that he had no remorse, saying those being held were "terrorists".

Seif Saad, 18, a former labourer with no formal police training, described how security forces raided homes of the detained suspects or snatched them from the streets.

"We placed sacks on their heads and tied their hands behind their backs," he told Reuters.

"They were brought here for interrogations over bombings. Some were released. Others were held or taken to prisons."

An Iraqi cleric who said that he was familiar with the complex said more than 40 men were found in one cell measuring 4m by 5m (12ft by 15ft). "Some of the prisoners looked frail. Medics were brought in. They were sick and they talked about how they were tortured," he told Reuters.

Hussein Kamal, Iraq’s deputy interior minister, told CNN that he saw evidence of torture: "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 body."

The condemnation from the Sunni community was echoed around the world. Jack Straw, the Foreign Secretary, spoke of his "deep shock" and said: "Whoever carried out this abuse must be caught and brought to justice, regardless of rank or background."

The US embassy issued a statement welcoming the Iraqi government’s decision to investigate the case and stressing that the mistreatment of detainees was "totally unacceptable".

The revelations came just a month ahead of a general election to elect a permanent government in Iraq, the final stage in its uneasy transition to democracy following the removal of Saddam.

An independent review is considered crucial by Sunni Arabs, who have repeatedly warned that Iraq’s security agencies have become infiltrated by Shia militiamen, who engage in torture and extra-judicial executions. The Iraqi Islamic Party said today that it was clear that Sunnis were being targeted ahead of the parliamentary elections.

Tariq al-Hashimi, the secretary-general of the Iraqi Islamic Party, held up photos of the bodies of people who appear to have been subjected to torture and said: "This is what your Sunni brothers are being subjected too."

Saad Farhan, a 40 year-old trader in Ramadi, said his brother and cousin were detained by Interior Ministry forces. He said: "Some government officials want to keep the Sunnis away from the next elections by terrorizing us. We believe that Iran’s agents are behind it because a normal and genuine Iraqis never do this."

The US Army was today facing further claims of torture after two Iraqi businessmen reported that they were put into a cage with lions.

Donald Rumsfeld, the Defence Secretary, said that the accusation - made by Sherzad Khalid and Thahe Sabbar as part of a lawsuit against US military commanders filed by the American Civil Liberties Union - was "far-fetched". When questioned further, he refused to deny it had taken place and said that an inquiry would be held.

In an indication of growing cross-party unease over the US presence in Iraq, the Republican-led Senate last night rejected a Democrat call for a specific timetable for the withdrawal of troops but demanded that 2006 be a "significant transition" year in which Iraqi forces took the lead in securing their country.

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