A homicide car bomber rammed a truck carrying Shiite pilgrims returning from a major religious commemoration Sunday, killing at least 32 people a day after Iraqi leaders warned sectarian violence could eventually spread around the region.

The truck was among the convoys ferrying Shiites home from Karbala in southern Iraq, where millions gathered over the weekend — where they also mourned the deaths of hundreds of pilgrims killed by suspected Sunni extremists last week heading for the ceremony.

Attacks on other pilgrim vehicles killed at least five people in Baghdad. In northern Iraq, a homicide bomber attacked the offices of Iraq's biggest Sunni political party, killing three guards.

In central Baghdad, the truck was bringing about 70 men and boys home when it was blasted by the car bomber. At least 24 people were injured, police and hospital officials said.

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One of the pilgrims, Nasir Sultan, said he was thrown from the truck by the blast. "I could see lots of burned bodies," he said.

Another pilgrim, Mustafa Moussawi, a 31-year-old vegetable store owner, said they group felt safe after crossing from Sunni-dominated areas and reaching central Baghdad.

"Then the car bomber slammed us from behind," said Moussawi, who suffered injuries to his right hand and shoulder. "I blame the government. They didn't provide a safe route for us even though they knew we were targets for attack."

Iraqi security officials have struggled to protect the annual pilgrimage to mark the end of 40 days mourning for the 7th century battlefield death of the Prophet Muhammad's grandson. Shiites consider him the rightful heir of Islam's leadership, which cemented the rift with Sunni Muslims.

More than 300 Shiite pilgrims were killed by suspected Sunni bombers and gunmen as they streamed toward Karbala, about 50 miles south of Baghdad.

Iraq's prime minister, Nouri al-Maliki, appealed Saturday for international aid to fight sectarian bloodshed to open a gathering of neighbors and world powers, which included rare diplomatic exchanges between the United States and Iran.

He told delegates that Iraq's strife could spill across the Middle East if not quelled.

It resumed even as the delegates packed up the briefcases and dossiers.

The attack on the Iraqi Islamic Party's office came as politicians were leaving a reception, said party member Mohammed Shakir al-Ghanam. Three guards were killed and two were wounded, he said.

The reason for the attack was not immediately clear. The party is the only Sunni political movement with a national base.

Mosul, about 225 miles northwest of Baghdad, also has witnessed a rise in suspected Sunni insurgent attacks, including a reported raid on a prison last week that allowed nearly 150 prisoners to escape. Most were quickly captured.

In central Baghdad, a bomb-rigged car killed at least three pilgrims and injured six.

At about the same time, a homicide bomber detonated a belt packed with metal fragments inside a minibus heading to a mostly Shiite area, killing at least 10 people and wounding five.

The attacks followed a homicide car bombing Saturday in Baghdad's main Shiite militia stronghold, Sadr City. The blast at a checkpoint killed 20 people, including at least six Iraqi soldiers.

But it carried additional worries for U.S.-led forces, who entered Sadr City last week under a carefully negotiated deal with political allies of the Madhi Army militia, led by radical cleric Muqtada al-Sadr.

He has agreed to withhold his armed militia from the streets during a U.S.-Iraqi security crackdown begun last month. But attacks on his power base could encourage al-Sadr to send his fighters back to protect Shiites if U.S.-Iraqi forces cannot.

They could also rekindle tit-for-tat sectarian killings that have receded since the Baghdad security push began nearly four weeks ago.

The U.S. military said that American and Iraqi troops had captured three suspected members of a bomb-making cell north of Baghdad.

The men were detained during an air assault Saturday in Tarmiyah, the military said in a statement. The suspects were accused of planting roadside bombs and car bombs in attacks on American and Iraqi troops, it said.

The military also announced the capture of a suspected financier of insurgents in Kirkuk province, who was taken into custody last Wednesday, another statement said.

Also Sunday, a roadside bomb killed two women in a civilian car in Mahmoudiyah, 20 miles south of Baghdad, police said.

Complete coverage is available in FOXNews.com's Iraq Center.