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A suicide attacker detonated a car bomb Friday alongside a crowd of Shiite pilgrims walking to a holy city south of Baghdad, killing at least 31 people and wounding 150, a security source said.
Shiite pilgrims have been targeted by several bombings in recent days, and Friday's attack struck during the culmination of a pilgrimage in which hundreds of thousands walked to the city of Karbala to mark a Shiite holy day.
The blast occurred shortly after noon just east of one of three main entrances to Karbala, in the Qantarat al-Salam district, said the source.
Two mortar rounds hit the same area after the car bomb exploded, the official said, adding that the death toll was likely to rise.
The attack came at the height of the pilgrimage when the roads around Karbala, 50 miles south of Baghdad, were clogged with people trying to reach the city by Friday, another police official said.
The crowds made it difficult for ambulances to get to the wounded, he said.
All the officials spoke on condition of anonymity because they were not authorized to release the information.
The Arbaeen holy day marks the end of 40 days of mourning after the anniversary of the death of Imam Hussein, a revered Shiite figure.
The concentration of Shiites and the difficulty of securing the crowds streaming along major roads, make the ceremony a prime target for suspected Sunni militants. In another attack Friday, a roadside bomb struck a bus carrying pilgrims through Baghdad, killing one and wounding 13, police and hospital officials said on condition of anonymity for the same reason as the others.
The attack in Karbala was just a short distance from where a motorcycle bomb exploded two days earlier, killing dozens. On Monday, a female suicide bomber killed at least 54 pilgrims in an attack just north of Baghdad.
Iraqi security forces have increased protection for pilgrims but face huge challenges trying to find a single attacker in the crowds.
This week's violence took place as Iraqi politicians argued over an effort to bar hundreds of candidates from running in the March 7 parliamentary elections because of suspected ties to Saddam Hussein's regime. Prime Minister Nouri al-Maliki said Thursday he would not allow the U.S. ambassador to meddle in the dispute, which Washington fears could frustrate Sunni-Shiite reconciliation.
Tension also escalated this week between al-Maliki's Shiite-led government and Iraq's Sunni politicians over the push to ban some candidates from next month's election.
A parliamentary committee responsible for rooting out Saddam loyalists blacklisted more than 450 politicians, but an appeals court overturned the ban on Wednesday.
The U.S. is deeply worried the ban could undercut the credibility of the election among Iraqis and cripple efforts to reconcile majority Shiites and the Sunnis who dominated Iraq under Saddam.
U.S. Ambassador Christopher Hill applauded the decision to lift the ban and has said that Iraq must have a credible election.
Al-Maliki warned Hill not to get involved.
"We will not allow American Ambassador Christopher Hill to go beyond his diplomatic mission," al-Maliki said late Thursday in a statement published on his political coalition's Web site.
Al-Maliki said the ban on the candidates should be implemented and that Iraq must not bow to U.S. pressure.
The U.S. Embassy dismissed the warning, saying that Hill has been doing what any diplomat normally does - offering his government's views on issues that could affect American interests.
"That is not going beyond the bounds of acceptable diplomacy. Iraqi leaders take on board our views but then make their own decisions," said an embassy statement issued to The Associated Press. "Of course, we respect Iraqi sovereignty."