Twin car bombs tore through a crowd of Shiite pilgrims packing a highway as they walked to a holy city south of Baghdad on Friday for a major religious observance, killing at least 40 people and wounding 154 others, Iraqi officials said.

It was the third deadly bombing this week hitting the ceremony in which hundreds of thousands of Shiites have been converging on the city of Karbala. Friday's attack struck during the culmination of the pilgrimage.

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 former 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 hamper Sunni-Shiite reconciliation.

Friday's attack began shortly after noon when a parked car bomb exploded just east of one of three main entrances to Karbala, two Health Ministry officials said. The explosion sent throngs of pilgrims running down the highway and straight into the path of a suicide car bomber who detonated the vehicle, they said.

At least 154 people were wounded in the consecutive blasts, the officials said.

However, an Iraqi police official reported it was two mortar rounds that struck the area, followed by a suicide car bomb. Such conflicting accounts are common in the chaotic aftermath of bombings in Iraq.

The attack came at the height of the pilgrimage when roads around Karbala, 50 miles south of Baghdad, were clogged with people trying to reach the city by Friday. The crowds made it difficult for ambulances to get to the wounded, another police official said.

The officials all spoke on condition of anonymity because they were not authorized to release the information.

The Arbaeen holy day, preceded by days of mass marches to Karbala, 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 makes the annual ceremony a prime target for suspected Sunni militants.

Iraqi security forces have increased protection for pilgrims but face huge challenges trying to find a single attacker in the crowds — and this year's Arbaeen commemorations have been the deadliest since 2007.

Friday's twin bombing in Karbala was just a short distance from where a bomb exploded two days earlier, killing around two dozen people. And on Monday, a female suicide bomber killed at least 54 pilgrims heading for the city in an attack just north of Baghdad.

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.

In each of the past two years, attacks during the ceremonies killed around 60 Shiites, a drop from the more than 340 killed in 2007.

Iraqi security forces have increased protection for pilgrims but face huge challenges trying to find a single attacker in the crowds.

But Ali Ahmed, a 32-year-old Shiite bookshop owner in Baghdad, blamed Iraq's authorities for failing to ensure security for the faithful.

"The huge number of pilgrims and the lack of experience of the Iraqi security forces led to these explosions," he said.

In Pakistan, where Shiites have also faced attacks by Sunni extremists, two bombs targeting those observing Arbaeen exploded on Friday. One blast went off outside a hospital treating victims from an earlier attack on worshippers heading to a procession. At least 25 people were killed and more than 100 wounded.

In Iraq, 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.

Al-Maliki denounced the ruling, and election officials have asked Iraq's highest judicial authority for a final ruling.

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," Philip Frayne, an embassy spokesman in Baghdad, said in an e-mail to The Associated Press. "Of course, we respect Iraqi sovereignty."