Twenty-five people were killed and 150 were wounded on Saturday afternoon in a truck bombing near a Shi'ite mosque in northern Iraq, police said.
Brig. Gen. Sarhat Qader said the explosion occurred following noon prayers south of the disputed city of Kirkuk.
The blast comes as US troops have begun withdrawing from urban areas ahead of a June 30 deadline set by a security pact.
Iraqi officials have warned they expect insurgents to stage attacks to try to undermine confidence in the government's ability to take over its own security.
Prime Minister Nouri al-Maliki has insisted the US troops withdraw from the cities on schedule, saying it will be a "big victory" for the Iraqi people.