Two suicide bombers have killed at least 40 people and injured more than 140 on the outskirts of the Iraqi city of Karbala, police reports say.
About a million Shia Muslim pilgrims are in the city to visit the Imam Hussein shrine. About 60 pilgrims were killed in two other attacks this week.
Friday is the last and most important day of the Arbaeen, 40 days of mourning for the Prophet Muhammad's grandson.
Meanwhile, 25 people were killed in an attack on Shias in Karachi in Pakistan.
Police said the Karbala attack was a double suicide bombing - two cars packed with explosives were detonated on either side of a bridge across which pilgrims were making their way in and out of the city.
Some reports said the car bomb attack was followed up by mortar rounds.
Provincial governor Amalheddin al-Hir told the AFP news agency he believed that al-Qaeda militants, supported by the outlawed Baath party of former leader Saddam Hussein, had carried out the attack.
The attack came despite heightened security for the pilgrimage to Karbala, which Mr Hir said had drawn about 10 million worshippers to the Imam Hussein shrine over the past two weeks.
This year's pilgrimage has already been hit twice by bombers.
A bomb planted on a cart pulled by a motorbike killed at least 20 pilgrims on Wednesday as they streamed into Karbala.
And more than 40 pilgrims were killed on the outskirts of the capital Baghdad on Monday as they began the long walk to Karbala.
The bombings also raise fears of an increase in sectarian violence before Iraq's March parliamentary elections.
The tension has risen with the row over the banning of more than 500 candidates, many for links to the Baath party.
Sunnis believe they have been heavily targeted in the exclusions, which are the subject of a legal challenge.
The BBC's Gabriel Gatehouse in Baghdad says that the stakes are high; a peaceful and credible election would allow the country to draw a line underneath the bloodshed and turbulence of recent years, he says.
But, he adds, these recent bombings have raised fears of a return to sectarian violence, just as American forces prepare to withdraw.
The Pakistan attack, possibly involving a bomb on a motorcycle, targeted a bus carrying Shia Muslims to a religious procession also marking Arbaeen.
In addition to the 11 dead, about 50 people were injured.
The Shia-Sunni schism originates from a dispute soon after the death of the Prophet Muhammad over who should lead the Muslims.
Sunnis remain the majority globally, with Shias estimated to number about 10% of all Muslims. Shias are however the majority in Iraq.
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