In all, seven vehicles rigged with explosives went off in five cities south of Baghdad during morning rush hour, leaving 56 people wounded in primarily Shiite Muslim areas of Iraq.
No group immediately claimed responsibility for the attacks, but Sunni militant groups linked to Al-Qaeda frequently target Shiites, whom they regard as apostates, in coordinated attacks.
Car bombs went off in Kut, Aziziyah, Mahmudiyah, Nasiriyah and Basra, officials said.
Another car bomb in nearby Aziziyah in the town's main marketplace and near a Shiite mosque killed five and wounded 10.
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Twin blasts in the southern port city of Basra, meanwhile, killed five people, including a bomb disposal expert looking to defuse one of the rigged vehicles.
Three others were killed in bombings in Nasiriyah and Mahmudiyah.
The violence was the latest in a spike in attacks nationwide, with last month registering the highest death toll since 2008, sparking fears of a return to the all-out sectarian war that blighted Iraq in 2006 and 2007.