A religious procession by hundreds of thousands of Shiites in Baghdad was marred Sunday by sniper attacks that killed 20 pilgrims, a tragic reminder of the deep sectarian divisions in a country that many fear is heading toward all-out civil war.

About 300 people were injured, mostly when they fell while running to escape the snipers firing from buildings in Sunni dominated areas along the procession routes. In one neighborhood, gunmen hid behind tombstones at a Sunni cemetery.

CountryWatch: Iraq

"I was walking and someone got shot in front of me. It wasn't random fire, it was a clear sniper attack," said Mohammed Jassim, 32.

He said he could hear the faint crack of the shots despite the noise from the procession. "People panicked and started yelling 'it came from here, no from there."'

The violence occurred despite unprecedented security and a weekend driving ban to prevent car bombs amid a cycle of tit-for-tat attacks by Shiites and Sunnis since the Feb. 22 bombing of a Shiite mosque in Samarra.

Still, the main ceremonies at the golden-domed shrine to Imam Moussa Kadhim, one of 12 Shiite saints, continued peacefully.

The attacks on the pilgrims took place in three or four neighborhoods at least 1 mile away from the shrine in Kazimiyah in north Baghdad, where Kadhim, who died in 799 during a Sunni caliph's rule, is buried.

"We heed your call, Oh Imam!" the pilgrims intoned before entering the compound, beating their chest and flagellating themselves with steel chains in a traditional Shiite expression of grief.

Fadhil al-Sharaa, an aide to Shiite Prime Minister Nouri al-Maliki claimed some 1 million people attended the procession, but other officials were more conservative, putting the figure at between 200,000 and 300,000.

The Health Ministry and the Interior Ministry gave identical casualty figures in the shootings: 20 pilgrims killed and 304 injured, only a few of them by bullets. Police also said four militants, including two snipers, were killed by security forces.

The U.S. military has received confirmed reports of five Iraqi civilians killed and is checking reports of about 20 killed, military spokesman Lt. Col. Barry Johnson said.

"Iraqi forces reacted immediately to each reported incident, suppressing attacks and detaining several suspects," he said.

About 12,000 extra U.S. and Iraqi troops have been deployed in recent weeks as part of a security crackdown in the capital as many fear an all-out civil war due to the sectarian bloodshed, which is claiming about 100 lives a day, and the Sunni Arab insurgency.

But Iraq's Industry Minister Fowzi Hariri refused to accept that a civil war was imminent.

"Sectarian wars happen in Rwanda. That's not happening in Iraq. The vast majority of the people of Iraq are determined to end up in a national unity country and a government that is for the whole of Iraqis," Hariri said on CNN's Late Edition program.

He acknowledged that "certain elements" are trying to destabilize the country and taking advantage of some "gaps" in the security system. "But they will not win," he asserted.

Last year, the government said about 1,000 people died during the Imam Kadhim commemoration when rumors of suicide bombers triggered a mass stampede on a bridge across the Tigris River. It was the biggest single day death toll since the U.S.-led invasion in March 2003.

On Sunday, reporters saw women in black "abayas," an Islamic female gown, helping each other as they ran for cover when the shooting started.

Many took refuge under an overpass, ducking in fear at the sound of gunshots. A cleric in a dark gray robe and white turban, holding a gun, was seen being pushed away from the scene by a security personnel.

The Iraqi Islamic Party, the main Sunni party, accused government forces and armed militias, a reference to Shiites, of killing "peaceful people," raiding homes and attacking Sunni mosques.

"We call on the government to adopt a strict stand against anybody who tries to create chaos and disorder," said a party statement.

Shiites were prevented from mustering huge crowds at religious ceremonies during Saddam Hussein's Sunni-dominated regime. But since Saddam's ouster in 2003, Shiite politicians and religious leaders have encouraged huge turnouts as a demonstration of the majority sect's power.