A female suicide bomber blew herself up among a crowd of Shiite pilgrims on their way by foot to a shrine city in central Iraq on Monday, killing 41 people including women and children.
Officials said the attack that also wounded 106 people targeted a rest station where pilgrims had stopped on their long journey to Karbala, 110 kilometres (70 miles) south of Baghdad, for a religious festival.
"At 11:45 am (0845 GMT), a woman wearing an explosives-filled belt blew herself up in the middle of a crowd of pilgrims going to Karbala," said Major General Qassim Atta, spokesman for Baghdad operational command.
An interior ministry official gave the toll and said the wounded were being treated at five hospitals in the capital.
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At least five women and six children were among the dead, a medical official said earlier.
"We were serving the people when the attack occurred inside a search tent for women," said Allawi Hassan, who was being treated at Kindi hospital in Baghdad.
"The moment the explosion happened I felt as if I was flying through the air. I saw men, women and children wounded before I fainted. I then found myself in hospital," added Hassan, whose legs were hit by shrapnel.
The victims had been travelling on foot from the central province of Diyala to Karbala to observe Arbaeen rituals.
Prime Minister Nuri al-Maliki's office in a statement blamed the Baath party of executed dictator Saddam Hussein for the attack.
"We hold Baathists and their Takfiri allies responsible for this massacre," it said.
Takfiri is a term used by the Iraqi government to refer to Al-Qaeda members.
Defence ministry spokesman General Mohammed al-Askari said the bomber came from Diyala province, northeast of Baghdad, that has in the past been a stronghold of Al-Qaeda which still has a local presence.
"Apart from today, there have already been 25 other attacks carried out by women suicide bombers coming from the province," he told AFP.
Arbaeen marks 40 days after the Ashura anniversary that commemorates the killing of revered 7th century Imam Hussein, whose shrine is considered one of the holiest places in Shiite Islam.
Tens of thousands of Shiites, including many from neighbouring Iran, make their way at Arbaeen to pay homage at the Karbala shrine, walking as a sign of greater piety.
Pilgrims are routinely searched at transit food stations because of the risk of attacks, with women coming in for special scrutiny as their faces are concealed.
Around 30,000 members of the Iraqi security forces have been deployed to Karbala for the holy festival which culminates on Friday.
Iraqi politicians and US forces have warned of rising violence ahead of a general election on March 7, the second parliamentary ballot since the 2003 US-led invasion which ushered in a deadly insurgency.
Rebels had in past months appeared to have directed their attacks away from strikes on religious targets to government buildings in Baghdad.
This has prompted the army to urge government officials to change their travel itineraries and avoid high-risk areas in the capital, the Baghdad military command said on Monday.
It also said rebels had "invented high-capacity explosives" that cannot be detected by controversial bomb detectors imported from Britain, which has banned their sale abroad.
Baghdad has been gripped by a series of deadly attacks that targeted government buildings.
Last Tuesday, a suicide bomb attack at Iraq's forensics headquarters in Baghdad, claimed by an Al-Qaeda group, killed 18 people and wounded 80 others.
The same group, the Islamic State of Iraq, admitted responsibility for three deadly bomb attacks on Baghdad hotels the previous day, huge blasts that killed at least 36 people and wounded 71.
Nearly 400 people were killed and more than 1,000 wounded last year in coordinated vehicle bombings at government buildings, including the ministries of finance, foreign affairs and justice in August, October and December.
Germany and the European Union both condemned the latest bloodshed.
"This act of terrorism should not stop the reconciliation of Iraq," German Foreign Minister Guido Westerwelle said during a meeting with EU foreign policy supremo Catherine Ashton, who denounced "the terrible incident."