A suicide bomber killed 53 people in an attack in southern Iraq on Saturday.
The attack on the outskirts of the port city of Basra, left 137 wounded.
The violence was the latest in a spate of attacks in the two weeks.
The bombing killed 53 people and wounded 137, according to Riyadh Abdulamir, head of Basra province's health department. He said women and children were among the casualties, but did not give further details.
The death toll was the highest since similar attacks in Baghdad and southern Iraq killed 70 people on December 5, and the deadliest single attack in the country since March 2011.
It was condemned by UN envoy Martin Kobler, and described as "a crime targeting national unity" by parliament speaker Osama al-Nujaifi.
The attacker, who had been distributing cake and other food to pilgrims walking to the Khutwa Imam Ali, a site on the outskirts of Basra venerated by believers for its associations with a key figures of their faith, blew himself up near a security checkpoint.
"I saw a soldier take hold of the attacker to take him to the officer in charge," said Kadhim Nasser, who ran a nearby rest stop for pilgrims. "As he was pushing him, something happened and the soldier fell to the ground."
"Immediately, he blew himself up. When he did that, women and children were passing by. I saw dozens of women and children among the wounded," the 42-year-old added.
Some 35,000 police and troops were deployed to provide security throughout the rituals, with a further two brigades added to protect pilgrims heading home, said Lieutenant General Othman al-Ghanimi, who commands forces across central Iraq.
He said no incidents had been reported in Karbala.
Provincial health spokesman Jamal Mehdi said hospitals in the shrine city had treated 20,000 pilgrims but that there had been no deaths.
This year is the first Iraqi troops have been solely charged with security for Arbaeen since the US-led invasion of 2003. American troops, who previously aided with surveillance and reconnaissance, completed their pullout last month.