At least 32 pilgrims have been killed by a female suicide bomber south of Baghdad, Iraqi police say.
Sixty-five people were also injured in the attack in Iskandiriya, 40km (25 miles) south of Baghdad.
The blast targeted Shia pilgrims, many of whom have been travelling south to the city of Karbala to take part in an annual religious ceremony.
It is the third day of attacks on Shia pilgrims, with dozens killed in Baghdad and Mosul.
The Iskandiriya attack happened despite increased security along the route.
A police captain told AFP news agency that the bomber had hidden explosives under an abaya, the traditional head-to-toe black garment, and blown herself up among the crowd just after midday (0900 GMT).
The pilgrims had gathered at a tent along the route for food and drink when the bomber struck, officials said.
Most of the dead were reported to be women and children.
A doctor at Hilla General Hospital said most of the survivors had head and chest injuries, AFP reported.
Hundreds of thousands of Shia have been converging on Karbala in recent days, to celebrate Arbaeen - the end of symbolic mourning for the Prophet Muhammad's grandson, Imam Hussein, whose shrine is in the city.
In past years the pilgrimage has been the target of similar attacks, blamed on Sunni extremists bent on stirring up sectarian strife.
This year has been no exception: pilgrims have been attacked in Karbala itself and in Baghdad, but this latest explosion has been the deadliest so far.
It was a grim reminder that despite the considerable general improvement in security in Iraq there are still people out there bent on igniting sectarian passions - something many Iraqis had hoped was becoming a thing of the past, says the BBC's Jim Muir in Baghdad.
Iraqi militants have increasingly used women to carry out suicide attacks as they are less likely to be searched than men.
In 2007, there were eight suicide attacks by women; in 2008 there were 32, the US military says. In early January, a female bomber killed at least 35 Shia pilgrims in a blast near a Baghdad shrine.
Iraqi officials arrested an alleged militant recruiter last month.
Samira Jassim allegedly recruited more than 28 women to blow themselves up in various parts of Iraq.
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