A bomb killed at least 53 Shi'ite pilgrims near the southern port city of Basra today, an Iraqi official said.
It was the latest in a series of attacks during Shi'ite religious commemorations that have killed scores of people and threaten to increase sectarian tensions just weeks after the US withdrawal.
The attack happened on the last of the 40 days of Arbain, when hundreds of thousands of Shi'ite pilgrims from Iraq and abroad visit the Iraqi city of Kerbala, as well as other holy sites.
Saturday's blast occurred near the town of Zubeir as pilgrims marched toward the Shi'ite 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 Shia site on the edge of a mostly Sunni town in an otherwise mostly Shiite province.
There were conflicting reports on the source of the blast.
Al-Emarah said the explosion was caused either by a suicide attacker or a roadside bomb. But an Iraqi military intelligence officer who is investigating the attack said it was a roadside bomb, noting that the road from Basra to Zubeir being used by pilgrims had been closed to traffic. He spoke on condition of anonymity as he was not authorised to brief the media.
Basra hospital received 53 killed and 137 wounded after the blast, said Dr Riyadh Abdul-Amir, the head of the Basra Health Directorate. He said some of the wounded were in serious condition, and warned that the death toll may rise further.
The explosion came as Shi'ite pilgrims commemorate the climax of Arbain, which marks the end of 40 days of mourning following the anniversary of the death of Imam Hussein, a revered Shi'ite. Pilgrims who cannot make it to the holy city of Kerbala, south of Baghdad, often journey to other sacred sites such as the shrine near Zubeir.
Majid Hussein, a government employee, was one of the pilgrims heading to the shrine. He said people began running away in panic when they heard a loud explosion.
"I saw several dead bodies and wounded people, including children on the ground asking for help. There were also some baby strollers left at the blast site," he said.
The attack, which bore the hallmarks of Sunni insurgents, is the latest in a series of deadly strikes in this year's Arbain. More than 145 people have been killed.
The largest of the Arbain attacks – a wave of apparently co-ordinated 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 Shi'ite militias and others that brought the country to the edge of civil war in 2006.
But this wave of attacks comes at a particularly tense time.
The last US combat troops left the country on Dec 18. Many Iraqis resented the foreign presence, but the Americans also guaranteed the status quo. Many Sunnis fear being marginalised in the now Shi'ite-dominated country following the US departure.
Just as the American troops were leaving, a political crisis erupted that has paralysed Iraq's government. It pits the country's mostly ethnic- and religious-based political blocs against one another.
The political dispute appears far from being resolved.
On Friday, Deputy Prime Minister Saleh al-Mutlaq called for Iraq's leader, Prime Minister Nouri al-Maliki, to step down or face a parliamentary vote of no-confidence. Al-Mutlaq's Sunni-backed Iraqiya party has been 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.
Al-Maliki's government, meanwhile, has demanded the arrest of the country's top Sunni politician, Vice President Tariq al-Hashemi of Iraqiya, accusing him of running a hit squad targeting government officials. Al-Hashemi denies the allegations.