A suicide bomber blew himself up inside a Sunni mosque in central Iraq, killing at least 20 people in the middle of a sermon, police and medics said.
The explosion took place in the town of Wajihiya in the ethnically and religiously diverse province of Diyala, which has seen an increasing number of attacks in recent weeks.
It was not clear who was behind the blast, which appeared to be part of a sustained campaign of militant attacks that has prompted fears of wider conflict in a country where ethnic Kurds and Shi'ite and Sunni Muslims have yet to find a stable power-sharing compromise.
Sectarian tensions have been inflamed by the civil war in neighbouring Syria, which has drawn in Shi'ite and Sunni fighters from Iraq and beyond to fight on opposite sides of the conflict.
Sunni Islamist insurgents, including the al Qaeda-affiliated Islamic State of Iraq, have been regaining strength in recent months, security sources say.
More than 460 people have been killed in militant attacks in July so far, according to violence monitoring group Iraq Body Count.