The bloodshed came as a flood of worshippers, including tens of thousands of foreign pilgrims, thronged the central shrine city of Karbala for the climax of Ashura, braving repeated attacks by Sunni militants that have marred the festival in previous years.
The suicide bomber, disguised in police uniform, struck in a Shiite-majority area of confessionally mixed Diyala province, north of Baghdad, killing 32 people and wounding 80, security and medical officials said.
Shiites thronged Karbala to mark Ashura
It was the third attack of the day targeting Shiites. Earlier, coordinated blasts in Hafriyah south of the capital killed nine people, while twin bombings in the northern oil city of Kirkuk wounded five. Violence near Baghdad and in Diyala's provincial capital Baquba left three others dead.
Shiites from Iraq and around the world mark Ashura, which this year climaxed on Thursday, by setting up procession tents where pilgrims gather and food is distributed to passers-by.
An estimated two million faithful gathered in Karbala, site of the mausoleum of Imam Hussein, grandson of the Prophet Mohammed, whose death in the city at the hands of soldiers of the caliph Yazid in 680 AD lies at the heart of Islam's sectarian divide.
To commemorate the occasion, modern-day Shiite devotees flood Hussein's mausoleum, demonstrating their ritual guilt and remorse for not defending him by beating their heads and chests. In some cases they make incisions on their scalps with swords in ritual acts of self-flagellation.