The police said six people were detained after the shooting late Monday in Eastern Province, which came as Shias prepared to mark the peak of Ashura, one of the holiest festivals of their faith.
Nine other worshippers were wounded in the attack.
Three assailants fired machine guns and pistols at the crowd in the village of Al-Dalwa, the police told the official SPA news agency.
Meanwhile, a suicide bombing and shelling targeting Shias in Baghdad killed at least 16 people ahead of Ashura commemorations, Iraqi security and medical officials said on Tuesday.
Shelling struck a street in the Tunis area of northern Baghdad where Shias were distributing refreshments from a tent on Monday, after which a suicide bomber detonated explosives in the same area, a police colonel said.
