Supporters of a candidate for the Azimun Ala al-Bina (Determined to Build) party had gathered at around lunchtime in the restive city of Baquba, 60 kilometers (35 miles) north of Baghdad. As the meeting went on, a man threw a grenade before then a bomber blew himself up, a police colonel and a medic at Baquba hospital said.
At least 25 people were killed and 60 others were wounded, the officials said, though the candidate appeared to have been left unharmed.
Iraq has been plagued by attacks against election candidates ahead of provincial council polls due on April 20, with 12 hopefuls killed so far. The polls will be the latest in Iraq since a March 2010 parliamentary election.
Violence has spiked ahead of the provincial elections, with at least 271 people killed in March alone.