Car bombs hit four Shi'ite mosques in the Iraqi capital Baghdad and another in Kirkuk just after prayers on Friday, tearing into crowds of worshippers and killing 17, police and witnesses said.
Sunni Islamists linked to al Qaeda's Iraqi wing have stepped up attacks this year and often target Shi'ite sites in a growing sectarian confrontation a decade after the U.S.-led invasion.
Police said blasts hit Shi'ite mosques in southeastern and northern Baghdad and another in Kirkuk, the ethnically mixed city of Arabs, Kurds and Turkman 170 km (100 miles) north of the capital.
"We were listening to the cleric's speech when we heard a very strong explosion. Glass scattered everywhere and the roof partially collapsed," said Mohammed, a victim wounded in the Kirkuk blast, his shirt still covered in blood.
Attacks in Iraq are still below the worst Sunni-Shi'ite slaughter that erupted at the height of the war when insurgents bombed the Shi'ite al-Askari shrine in Samarra in 2006, provoking a wave of retaliation by militias.
But security officials say al Qaeda's wing, Islamic State of Iraq, is regrouping in the desert of western Iraq, invigorated by the war and flow of Islamist fighters battling against President Bashar al-Assad in neighbouring Syria.