A suicide bomber has killed at least 13 people at a Shi'ite mosque north of Baghdad, after three other attacks leave eight dead.
The attack during evening prayers left 25 people wounded.
Reuters reports that sectarian tensions in Iraq and the wider region have been inflamed by the civil war in Syria, where mainly Sunni Muslim rebels are fighting to overthrow President Bashar al-Assad, whose Alawite sect derives from Shi'ite Islam.
Insurgents including al Qaeda's Iraqi affiliate have been regaining ground and recruits from the country's Sunni minority, which feels sidelined since the US-led invasion toppled former dictator Saddam Hussein and empowered majority Shi'ites.
Scattered attacks across the country throughout the day killed at least 22 others, around half of them in or near the northern city of Mosul, where a suicide bomber killed four people at a police checkpoint.
In the western province of Anbar, which shares a border with Syria, militants detonated two car bombs near a checkpoint and attacked it with rocket-propelled grenades, killing five policemen.
Two people were killed when gunmen hurled a hand grenade at a gathering of labourers in Tikrit, 150 km north of Baghdad, and a roadside bomb near some restaurants in the centre of the capital killed two more.
More than 1000 people were killed in Iraq in May alone, making it the deadliest month since the sectarian bloodletting of 2006-07.