At least 25 police recruits have been killed in a suicide attack on a police headquarters in the city of Baquba, 60km northeast of Baghdad, police said.
The victims were waiting outside the building ahead of training on Monday when a bomber blew himself up in the midst of them, a police officer said.
The attack came as the US military prepared to hand over security of the central province of Karbala, 80km south of the capital.
The province, which is relatively peaceful compared to some other central and western regions, will be the eighth to be transferred to Iraqi control.
Brigadier General Kudair al-Timimi from the Baquba police said that 20 people, including a woman and child, were wounded in the early morning attack.
The city's police chief was among 26 people killed last month when a suicide bomber struck in a mosque compound where local Shia and Sunni Arab leaders were holding reconciliation talks.