Police found the corpses of 36 murder victims on Baghdad's streets Thursday, according to the US-led coalition, after what a military spokesman dubbed a "tremendous spike" in killings in the Muslim holy month of Ramadan.
Iraqi police said that another 14 corpses had been found in a plantation north of the capital, where gunmen had rounded up workers and shot them in the head.
Most of the murders are thought to have been carried out by the death squads that roam Iraq, killing with virtual impunity on behalf of the Sunni and Shiite factions vying for control.
According to US commanders, many of the estimated 23 militia groups in and around Baghdad are linked to or protected by Sadr's Mehdi Army, a loosely organized force of young Shiite gunmen.
But Friday, Sadr called on those who had killed Iraqis to "repent."
"There are rumors that there are groups or persons from the Mehdi Army attacking the Iraqi people with no right to do so," Sadr said in a statement bearing his signature distributed by his office in the shrine city of Najaf. "It is not proved so far but, if proved, I will declare their names and will renounce them with no fear or hesitation," he said.
Tens of thousands of young men have flocked to the Mehdi Army banner in the three-and-a-half years since the overthrow of Saddam Hussein, giving the fiery preacher a powerful political and military voice.
In August 2004, Sadr's gunmen fought fierce battles with US forces in Najaf and in their Baghdad stronghold of Sadr City but their leader has since chosen to follow a more political route to power.
Nevertheless, in recent months black-clad fighters claiming allegiance to the Mehdi Army have again been fighting US and Iraqi forces and have been accused of supporting death squads.
US commanders now cite Shiite militias as the biggest single threat to the stability of Iraq and say that they are awaiting the green light from Prime Minister Nuri Al Maliki to launch an operation to clear Sadr City of gunmen.
"Criminals should not take righteousness as a shield," Sadr said, warning that if they continue to fight he will no longer seek to protect them. "I ask them to do this because I love them, not because I need them."
Iraq's slaughter continued Friday, when a bomb exploded in the office of Colonel Salaam Maamuri, commander of a US-trained police rapid intervention team in the mainly Shiite central city of Hilla, killing him and his deputy.
Maamuri has survived several previous assassination attempts. His squad was set up with US support to fight insurgents in an area just south of the capital that has become notorious as the "Triangle of Death."
Meanwhile, a curfew was imposed in the northern city of Mosul after fierce overnight clashes left eight insurgents and one police lieutenant dead, police Colonel Abdel Karim Al Juburi said.
There was another massive security operation in the Shiite shrine city of Najaf at the start of several days of ceremonies to mark the martyrdom of the sect's first leader, the Imam Ali, who was killed in the year 661.
Violence is spiraling in Iraq, where overlapping sectarian, political, and ethnic conflicts combined with a rebellion against the US-backed government and US-led forces claim more than 100 lives a day.
Against this background, British army chief of staff General Richard Dannatt said in a newspaper interview Friday that Britain's 7,200 troops in the south of the country were "exacerbating" the situation and should be pulled out.
Dannatt said that Britain should "get ourselves out sometime soon because our presence exacerbates the security problems."
In other attacks Friday, three Shiite youths - thought to be Sadr supporters - were gunned down by a cigarette stand in the ethnically divided northern oil hub of Kirkuk, police Captain Imad Jassim Khudir said.
Kirkuk and its oilfields are claimed as part of Kurdistan by the leaders of Iraq's Kurdish minority, but Saddam flooded the area with settlers in a policy of forced Arabization, leading to ethnic tension.
Last week, 14,000 Iraqi troops conducted a broad security operation in the city with US support, but the violence continues. Gunmen also shot an off-duty Iraqi soldier in front of his home Friday, police said.
And in the southern city of Amara, a Shiite militia stronghold, a police corporal was shot dead inside his house, police said.
© 2006 Agence France-Presse