Police in the past 24 hours have found the bodies of at least 87 men killed by execution style shootings in a gruesome wave of apparent sectarian killing, the Interior Ministry said Tuesday. They include at least 29 bodies stacked in a mass grave in an eastern Shiite neighborhood.

Acting on an anonymous tip, police found a 6-by-8-meter (yard) hole in a empty field. It contained at least 29 dead men most of them in their underwear in Kamaliyah, a mostly Shiite east Baghdad suburb.

Local residents offered scarves to help cover the bodies, which were laid out on the ground. Police guarded the site as members of a Shiite militia dug for more corpses. An Associated Press photographer took pictures of the grave but was warned not to publish them.

At least 40 more bodies were recovered in Baghdad, including both Sunni and Shiite neighborhoods, said al-Mohammedawi.

They included four men shot in the head execution-style and hanged from electricity pylons in Sadr City, where two car bombs and four mortar rounds shattered shops and market stalls at nightfall Sunday, as residents shopped for food for their evening meals.

The scene, although gruesome, was not what many had feared: That the deadly explosions the previous night would ignite all-out civil war.

Radical Shiite cleric Muqtada al-Sadr whose stronghold was targeted Sunday refused to be provoked. With thousands of his Mahdi Army militiamen ready to fight, the anti-American leader called for calm and national unity.

Sunni religious leaders quickly condemned the assault on Sadr City.

Sheik Ahmed Abdul Ghafour al-Samaraie, head of the Sunni Endowment, the state agency responsible for Sunni mosques and shrines, called it "a cowardly and criminal act."

"There are some hands trying to add fuel to the fire for their own benefit," he said on television.

An American described as a security contractor arrested by police in a northern Iraqi town was carrying weapons in his car, a provincial official said.

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The Joint Coordination Center between the U.S. and Iraqi military in Tikrit said the man, whom it described as a security contractor working for a private company, possessed explosives which were found in his car. It said he was arrested on Tuesday.

Iraqi police detained an American private security contractor working at a U.S. military base in northern Iraq for several hours on Tuesday, a U.S. military spokesman said.

The spokesman said the man was arrested at a checkpoint in the northern town of Tikrit. He denied initial reports that explosives were found in the car, but said two AK-47 assault rifles were in the vehicle.

U.S. President George W. Bush said insurgents were trying to ignite a civil war by escalating the violence.

"I wish I could tell you that the violence is waning and that the road ahead will be smooth," Bush said in a speech at the Foundation for Defense of Democracies at George Washington University. "It will not. There will be more tough fighting and more days of struggle, and we will see more images of chaos and carnage in the days and months to come."