A bomb killed at least 53 Shiites near the southern port city of Basra on Tuesday.
The bombing was just the latest in a series of attacks during Shiite religious commemorations following the US withdrawal from Iraq less than two months ago.
According to Iraq health officials Basra hospitals received 53 killed and 137 wounded after the blast.
Witnesses said the attack occurred outside the town of Zubair, southwest of Basra, as worshippers made their way to a Shiite shrine nearby. A police official confirmed the death toll for reporters.
The governor of Basra province's spokesman, Eyad Al Emarah, said it was not clear whether the blast was caused by a suicide attacker or a roadside bomb. Zubair is a predominantly Sunni enclave in Iraq's largely Shiite south.
The explosion came as Shiites commemorate the climax of Arbaeen, which marks the end of 40 days of mourning following the anniversary of the death of Imam Hussain, a revered Shiite figure.
The attack, which bore the hallmarks of anti-regime Sunni terror-insurgents who have coordinated bombings in Baghdad, is the latest in a series of deadly strikes in this year's Arbaeen. Scores have been killed.
The largest of the Arbaeen attacks killed at least 78 people on January 5. It was the deadliest strike in Iraq in more than a year.
The attacks raise fears of a new sectarian rift that could destabilize the country now that US troops are gone.
Growing tensions between Sunni and Shiite politicians in Iraq's current unity government have resulted in deadlock.
Sunni politicians have accused Shiite Nouri al-Maliki of seeking to create a dictatorship while al-Maliki has in turned accused his detractors of hatching "assassination plots."