Women and children were among the pilgrims injured
At least 41 people have been killed and 106 injured by a female suicide bomber in north-east Baghdad, an interior ministry spokesman has said.
The woman detonated an explosives vest among a group of pilgrims making the journey to Karbala, 80km from Baghdad. A witness described seeing a fireball.
The Shia pilgrims were marking Arbaeen, the end of 40 days of mourning for the death of Prophet Muhammad's grandson.
Troops and police are on alert during the pilgrimage.
The bomber had hidden the explosives under her abaya - a long, black over-garment, Maj Gen Qassim al-Moussawi told the Associated Press news agency.
The attack happened as pilgrims walked through Baghdad
Witness Raheem Kadhom said pilgrims were "crying for help" as they lay injured on the ground in the Shaab neighbourhood, AP reported.
Others helped by taking them to hospitals in cars rather than waiting for ambulances, Mr Kadhom said.
Security has also been increased around the revered Imam Hussein shrine in Karbala during the annual festival, which culminates on Friday.
In recent years the pilgrimage has been the target of similar attacks, blamed on Sunni extremists.
Twelve months ago, 32 pilgrims were killed by a female suicide bomber near Baghdad.
Monday's attack comes amid heightened tension between Sunni and Shia groups ahead of parliamentary elections scheduled for March.
About 500 candidates have been barred from standing for office because of their alleged links to Saddam Hussein's outlawed Baath Party.
Although the list does include candidates from both sides, Sunni political groups feel disproportionately targeted by the exclusions.