Ten car bombs exploded across Baghdad province early on Sunday, killing at least 37 people, police and medics said.
Nine of the blasts hit predominantly Shi'ite Muslim districts, with the deadliest attack in the town of Nahrawan, south of the capital, where two car bombs exploded in quick succession near a busy market, killing seven people.
Shi'ites have been targeted by Sunni Muslim insurgents including al Qaeda who have been regaining momentum this year after being forced underground in 2007.
Insurgents have exploited growing anger among Iraq's Sunni minority, which complains it has been marginalised under the Shi'ite-led government that came to power following the U.S.-led invasion in 2003.
Sectarian tensions have been further exacerbated by the civil war in neighbouring Syria, which has drawn Sunnis and Shi'ites from Iraq and the wider region into battle against each other.
Violence, which reached a climax in 2006-07 before starting to ease, is now on the rise again, with around 3,000 civilians killed so far this year, according to monitoring group Iraq Body Count.