Three Shia Muslim pilgrims were killed by a roadside bomb as they walked to a holy site near the northeastern Iraqi city of Khanaqin on Tuesday, police said.
The Islamic State (IS) group claimed responsibility for the attack, the group's news agency Amaq reported.
The claim could not immediately be verified. The militants are active in the area and have targeted Shia pilgrims in the past.
Hundreds of thousands of Shia Muslims gather in the southern city of Karbala every year in Iraq for the annual pilgrimage of Arbaeen, which marks the end of a 40-day mourning period for the grandson of the Prophet Mohammad, Imam Hussein.
This year, the authorities estimated that more than 13 million worshippers had flocked to Karbala, 100 km south of Baghdad, from across Iraq, Iran, Lebanon, Pakistan, Afghanistan, the Gulf states and even Europe and the US.
Some 1.8 million visas were handed out to worshippers from neighbouring Iran.
To protect the Arbaeen commemoration 30,000 soldiers and police officers were deployed, as well as 22,000 fighters from the Popular Mobilisation Units paramilitary forces, commanders said.
Security forces checked pilgrims and vehicles and guarded hundreds of kilometres of roads leading to Karbala.
IS has waged a campaign of kidnappings and killings since its self-proclaimed caliphate in Iraq and neighbouring Syria collapsed in 2017.
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