Manfred Nowak said the situation in Iraq was "out of control", with abuses being committed by security forces, militia groups and anti-US insurgents.
Bodies found in the Baghdad morgue "often bear signs of severe torture", said the human rights office of the UN Assistance Mission in Iraq in a report.
The wounds confirmed reports given by refugees from Iraq, Mr Nowak said.
He told journalists at a briefing in Geneva that he had yet to visit Iraq, but he was able to base his information on autopsies and interviews with Iraqis in neighbouring Jordan.
"What most people tell you is that the situation as far as torture is concerned now in Iraq is totally out of hand," the Austrian law professor said.
"The situation is so bad many people say it is worse than it has been in the times of Saddam Hussein," he added.
Brutal methods
The UN report says detainees' bodies often show signs of beating using electrical cables, wounds in heads and genitals, broken legs and hands, electric and cigarette burns.
Bodies found at the Baghdad mortuary "often bear signs of severe torture including acid-induced injuries and burns caused by chemical substances".
Many bodies have missing skin, broken bones, back, hands and legs, missing eyes, missing teeth and wounds caused by power drills or nails, the UN report says.
Victims come from prisons run by US-led multinational forces as well as by the ministries of interior and defence and private militias, the report said.
The most brutal torture methods were employed by private militias, Mr Nowak told journalists.
The report also says the frequency of sectarian bloodletting means bodies are often found which "bear signs indicating that the victims have been brutally tortured before their extra-judicial execution".
It concludes that torture threatens "the very fabric of the country" as victims exact their own revenge and fuel further violence.
Mr Nowak said he would like to visit Iraq in person, but the current situation would not allow him to prepare an accurate report, because it would not be safe to leave Baghdad's heavily guarded Green Zone where the Iraqi government and US leadership are situated.
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