At least 90 Shia pilgrims have been killed and more than 150 wounded in a double suicide bombing in the central Iraqi town of Hilla, police said.
The pilgrims were heading from Hilla to Karbala
Two bombers wearing explosive vests blew themselves up in a large crowd.
It was the deadliest in a number of attacks against pilgrims heading to the city of Karbala for a religious event.
Iraq has seen a sharp rise in violence between Sunni and Shia Muslims since an attack on a key Shia shrine in Samarra just over a year ago.
Baghdad has been at the centre of a three-week-old US-Iraqi security push, and US military commanders have been warning that militants may focus their efforts on launching attacks outside the capital.
3 Feb 2007: 130 killed in lorry bomb in Baghdad's market in mainly Shia area
2 Dec 2006: More than 50 killed in car bombs in same Baghdad market
23 Nov 2006: 200 killed in wave of car bombings and mortar blasts in Baghdad's Shia Sadr City
7 April 2006: 85 killed in triple suicide bombing at Shia mosque in Baghdad
Some commentators have suggested that the Baghdad security push, which has weakened Shia militias, has had the effect of leaving them unable to protect their communities from Sunni militants, the BBC's Andrew North reports from Baghdad.
In other attacks on pilgrims, at least 10 people were killed in car bombings and shootings in Baghdad while three were killed in two separate shootings in Latifiya, to the south.
The attack in Hilla happened in the late afternoon.
At one local hospital, Dr Mohammed Timini told AFP news agency: "Among the wounded, there are 50 in a critical condition. Eighty percent of the casualties are young men, but there are women and children among the dead."
Arbaeen ends 40 days of mourning for Imam Hussein, grandson of Prophet Mohamed
It follows Ashura, which commemorates his slaying by Muslim rivals in 680
Imam Hussein's shrine is at Karbala
Shias were discouraged from visiting during Saddam Hussein's rule
The attack was one of the deadliest single incidents in Iraq.
The worst single bombing since the 2003 invasion was in Baghdad early last month, when at least 130 people were killed in a lorry bombing.
Thousands of pilgrims are heading to Karbala, 100km (70 miles) south of Baghdad, to attend the Arbaeen religious ceremony.
An Iraqi woman taking part in the pilgrimage, Eman Hussein, said news of the Hilla attacks had given pilgrims a greater determination than before to reach Karbala, her son told the BBC News website from London.
Arbaeen marks the end of 40 days of mourning for Prophet Muhammad's grandson, Imam Hussein, who was killed along with his family in 681, by the Muslim ruler of Arabia, Yazid.
The day is one of the most solemn in the Shia calendar.
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