The crackdown has driven fighters out of Baghdad into surrounding towns and cities, especially into Diyala province, north of Baghdad but also Salaheddin, where they have launched attacks on US and Iraqi forces and civilian targets.
After the attack, a curfew was imposed on Albu-Ajeel.
Many police were initially trapped under rubble, including one officer who called for help on his mobile phone, police said.
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While several police and hospital sources said only one truck bomb was used, another police official said a second vehicle, a car, was believed to have exploded just after the truck bomb detonated.
One police official said an officer who was trapped under rubble made a frantic call to the Tikrit police operations room.
"We are under the building, which collapsed on us. We think it was a car bomb," the official quoted the officer as saying.
The bombing in Tikrit came a day after a suicide truck bomber killed 12 soldiers at an Iraqi army checkpoint in Hilla, south of Baghdad.
And in overnight violence in Baghdad, three people were killed and 17 wounded during clashes between US forces and fighters in the New Baghdad district, police and witnesses said.
They said the clashes broke out after US forces were shot at while raiding an area near an office of the Shia leader Muqtada al-Sadr.
The security crackdown in Baghdad involves more than 20,000 extra U.S. troops.