ZUBAIR, Iraq (AP) ― A bomb tore through a procession of Shiite pilgrims heading toward a largely Sunni town in southern Iraq on Saturday, killing at least 53 people in the latest sign of a power struggle between rival Muslim sects that has escalated since the American military withdrawal.

Fears of more bloodshed have risen in recent weeks, with the U.S. no longer enjoying the leverage it once had to encourage the two sides to work together to rein in extremists. Most of the latest attacks appear to be aimed at Iraq’s majority Shiites, suggesting Sunni insurgents seeking to undermine the Shiite-dominated government are to blame.

Saturday’s blast happened on the last of the 40 days of Arbaeen, when hundreds of thousands of Shiite pilgrims travel to the Iraqi city of Karbala and other holy sites. The end of Arbaeen is one of the most sacred times for Shiites, and public processions to commemorate it were banned under Saddam Hussein.

The blast occurred near the town of Zubair as pilgrims marched from the nearby port city of Basra toward the Imam Ali shrine on the outskirts of the town, said Ayad al-Emarah, a spokesman for the governor of Basra province.

The shrine is an enclave within an enclave ― a Shiite site on the edge of a predominantly Sunni town in an otherwise mostly Shiite province.

There were conflicting reports of what caused the blast, with some officials saying a roadside bomb was to blame.

But witnesses at the scene described the perpetrator as a suicide bomber disguised as a volunteer handing out juice and food to pilgrims. Ali Ghanim al-Maliki, the head of the Basra provincial council, corroborated that account in an interview with Iraqiya state television.

Arbaeen marks the end of 40 days of mourning following the anniversary of the death of Imam Hussein, a revered Shiite figure who is the grandson of the Prophet Muhammad.

Pilgrims who cannot make it to Imam Hussein’s grave in the holy city of Karbala, south of Baghdad, often journey to other sacred sites such as the shrine near Zubair. “I saw several dead bodies and wounded people, including children on the ground asking for help. There were also some baby strollers left behind at the blast site,” said Majid Hussein, a government employee, who was one of the pilgrims heading to the shrine.

At least 53 people were killed and more than 130 wounded in the blast, said Dr. Riyadh Abdul-Amir, the head of Basra Health Directorate.

The U.S. Embassy strongly condemned the attack, saying such acts of violence “tear at the fabric of Iraqi unity.”

Many pilgrims were undeterred, and continued on the bloodstained road despite the explosion. Shoes and slippers, as well as the remains of abayas, the long black cloaks most women wear in public, littered the side of the road.

The attack bore the hallmarks of Sunni extremists, who believe Shiites are not true Muslims. It was the latest in a series of deadly strikes during this year’s Arbaeen.

More than 145 people have been killed in attacks seen to be aimed at Shiites since the start of the year.

The largest of the Arbaeen attacks ― a wave of apparently coordinated bombings in Baghdad and outside the southern city of Nasiriyah ― killed at least 78 people on Jan. 5. It was the deadliest strike in Iraq in more than a year.

So far there has been little sign of the revenge attacks by Shiite militias that brought the country to the edge of civil war in 2006. The Shiite prime minister, Nouri al-Maliki, has tried with some success to bring the militias’ supporters into the political process, but many of their members retain their weapons and could again take up arms.

In the evening, a parked car bomb exploded near a security checkpoint in Saddam’s hometown of Tikrit, killing one policeman and wounding four, police said.

The latest violence comes at a particularly tense time.

The last U.S. combat troops left Iraq on Dec. 18. Many Iraqis resented the foreign presence, but the Americans also guaranteed the status quo.

Many of Iraq’s minority Sunnis, who dominated the government under Saddam’s dictatorship, now fear being marginalized in the now Shiite-led country following the U.S. departure. They also resent what they see as Shiite heavyweight Iran’s meddling in the country’s domestic affairs.

“The whole situation is very tense. Sectarianism is coming back in force in this country,” said Ayad Allawi, a secular Shiite who heads the Sunni-backed Iraqiya party, in an interview with CNN’s “Fareed Zakaria GPS” set to air Sunday. “Iraq is passing through the most dangerous phase through its history now,” he added.

Just as the American troops were leaving, a political crisis erupted that has paralyzed Iraq’s government, pitting the country’s mostly ethnic- and religious-based political blocs against one another.

The spat began when al-Maliki’s government called for the arrest of the country’s top Sunni politician, Vice President Tariq al-Hashemi, accusing him of running a hit squad targeting government officials. Al-Hashemi denies the allegations.

Al-Hashemi’s Iraqiya party, meanwhile, is boycotting parliament and Cabinet meetings since last month to protest what it sees as efforts by al-Maliki to consolidate power, particularly over state security forces.

On Friday, Deputy Prime Minister Saleh al-Mutlaq of Iraqiya called on al-Maliki to step down or face a parliamentary vote of no-confidence. He accuses the prime minister of creating a new dictatorship.

Iraq’s Kurdish president, Jalal Talabani, condemned the Zubair attack as an effort by terrorists to undermine efforts to “heal the rift” dividing the country’s parties.

American officials have been pushing Iraq’s squabbling factions to resolve their differences in a way that will benefit all Iraqis. But Washington’s influence has been seriously diminished now that American troops are gone.

Deputy Secretary of State William Burns and other U.S. officials met with al-Maliki on Saturday. The State Dept. had said Burns would encourage Iraqi politicians to resolve their differences.

A statement released by al-Maliki’s office about their meeting made no reference to the domestic political crisis, focusing instead on relations between the two countries and Iraq’s neighbors.