At least 53 Shiite pilgrims were killed Saturday in a suicide bombing in the southern Iraqi city of Basra, according to medical and security sources, reported dpa.
A further 137 people were injured in the attack, despite the deployment of thousands of security personnel to protect the pilgrims.
The victims included women and children, added the sources.
The bomber was wearing an explosive belt when he blew himself up targeting a crowd of people who were heading to a mosque, a security source in Basra, 420 kilometres south-east of the Iraqi capital Baghdad.
The bomber was handing out cookies to the visitors and blew himself up when a policeman approached him, said witnesses.
The crowd was visiting a holy shrine in the city before heading to the southern city of Karbala, where a major Shiite festival is being held, added the sources.
Millions of Shiites were Saturday gathering in Karbala to mark the peak of Arbaeen pilgrimage, which commemorates 40 days after the death of Imam Hussein bin Ali, the grandson of the prophet Mohamed.
The annual festival has been the target of deadly attacks blamed on al-Qaeda and radical insurgents in recent years.
Separately, three policemen were killed and four civilians injured in a bomb blast near Shiite pilgrims in Mosul, located some 400 kilometres north of Baghdad, according to security sources.
Few hours later, a lieutenant colonel was killed when a bomb went off targeting an army patrol in southern Mosul.