At least 26 people are reported to have died after a suicide bomber targeted mourners at a Shia Muslim mosque south of Baghdad, according to police and a doctor.
Sunday's blast, which collapsed the roof of Al-Hussein Mosque in the Musayyib area, also wounded 50 people.
It came just hours after at least 11 people were killed and dozens injured in a double suicide-car bombing in Erbil, the capital of Iraq's autonomous Kurdish region, an area usually spared the violence plaguing other parts of the country.
The victims of that attack are believed to be members of the Iraqi Kurdish security forces, known as asayesh.
A senior security official in Erbil said the first blast was a suicide car bomb, while an explosives-rigged ambulance was detonated when people rushed to check for casualties.
While other areas of Iraq are plagued by near-daily violence that kills hundreds every month, the three-province Kurdistan region has largely been spared.
Sunday's blasts were the first to hit Erbil since May 2007, when a lorry bomb exploded near the same asayesh headquarters, killing 14 people and wounding more than 80.
A spokesman for Nuri al-Maliki, Iraqi prime minister, said the attack may be linked to the civil war in neighbouring Syria, where Sunni opposition fighters have been battling Kurdish forces.
The Musayyib mosque bombing is just the latest in a series of sectarian attacks on mosques and funerals in central Iraq that have raised the fears of a return to the all-out Sunni-Shia violence that peaked in 2006-2007 and killed tens of thousands of people.
On Friday, bombs exploded near two Sunni mosques in Baghdad as worshippers left after prayers, killing six people.
Another bombing targeted Sunni mourners in Baghdad on September 23, killing 15 people, while an attack on a Sunni funeral killed 12 the day before.
Bombings targeting Shia mourners killed 73 people in Baghdad on September 21, and two blasts at a Sunni mosque north of the capital killed 18 a day before that.
The latest violence brings the death toll to more than 790 people in September and upwards of 4,600 this year, according to AFP news agency figures based on security and medical sources.