The former Iraqi prime minister, Iyad Allawi, has called for immediate action against human rights abuses.
Allawi is contesting next month's parliamentary elections
Such abuses are as bad today as they were under Saddam Hussein, Mr Allawi told Britain's Observer newspaper.
His comments come two weeks after 170 detainees were found at an interior ministry centre, some allegedly suffering from abuse and starvation.
Iraq's president dismissed Mr Allawi's allegations, saying his government did not accept the torture of prisoners.
The BBC's Jim Muir in Baghdad says Mr Allawi's remarks come as Iraq prepares for parliamentary elections next month, which he hopes could see him return as prime minister.
His comments are likely to heat up the election debate and will go down well within the Sunni community, our correspondent adds.
"People are doing the same as (in) Saddam Hussein's time and worse," Mr Allawi told the newspaper.
"It is an appropriate comparison. People are remembering the days of Saddam.
"These were the precise reasons that we fought Saddam Hussein, and now we are seeing the same things."
Militias are operating within the Shia-led government, torturing and killing in secret bunkers, he said.
Mr Allawi - who was displaced earlier this year by Shia factions - said the militias had infiltrated the police, and warned that their influence could spread throughout the government.
Mr Allawi said that if urgent action was not taken "the disease infecting [the interior ministry] will become contagious and spread to all ministries and structures of Iraq's government".
He also warned of the danger of Iraq disintegrating in chaos.
"Iraq is the centrepiece of this region," he said. "If things go wrong, neither Europe nor the United States will be safe."
Mr Allawi was Iraq's first interim prime minister, but he failed to win January's election which brought the current Prime Minister, Ibrahim al-Jaafari, to power.
He has since formed a coalition to run in next month's parliamentary elections.