Many more were wounded in the attack in Baquba – the second targeting Iraq’s security forces in two days.
On Tuesday, a suicide bomber killed some 60 people at a police recruitment centre in Tikrit, the hometown of former Iraqi leader Saddam Hussein.
Violence in Iraq has ebbed in recent years, but deadly attacks persist.
Both Baquba – 65km (40 miles) north-east of Baghdad – and Tikrit are within what is known as the Sunni Triangle, a stronghold of Iraq’s insurgency.
In a separate attack on Wednesday, a suicide bomber targeted Shia pilgrims travelling on foot from Baghdad to the holy city of Karbala, killing at least two people and injuring 15.
Timeline: Recent Iraq attacks
Bodies of bomb attack victims outside a hospital in Tikrit, Iraq, 18 January 2011
* 25 August 2010: String of attacks targeting Iraqi security forces and checkpoints across the country kill more than 50
* 19 September 2010: Series of bomb attacks in two neighbourhoods of Baghdad kill more than 20
* 31 October 2010: Botched hostage-taking at Our Lady of Salvation Syriac Catholic church in Baghdad kills 50
* 2 November 2010: Series of Baghdad bomb attacks kill 70 people
* 18 January 2011: Suicide bomb attack on Tikrit police recruitment centre kills at least 60
Most of those killed in Wednesday morning’s Baquba attack were police, officials said.
Two attackers were thought to have been involved. One stepped out of the ambulance and opened fire on guards at the entrance of the city’s special security police centre before the vehicle was driven into the compound and detonated, reports said.
More than 60 injuries were reported, and more people are said to be buried under rubble after the explosion caused a building to collapse.
“There are more bodies buried in the ruins,” a spokeswoman for Diyala’s governor told Reuters news agency.
Iraqi police and army recruiting centres are often targeted by suicide bombers.
Anti-Shia attacks had been feared ahead of next week’s commemoration of Arbain – a 40-day mourning period observed by Shia Muslims for Imam Hussein, a grandson of the Prophet Muhammad killed at the battle of Karbala in the 7th Century.
Security has been stepped up ahead of the climax of the Shia pilgrimage next week, when hundreds of thousands of pilgrims are expected to converge on Karbala.
Overall violence in Iraq has fallen sharply since the height of the sectarian killings of 2006-07, but shootings and bombings remain a daily occurrence.
US forces formally ended their combat operations last August, ahead of a planned full withdrawal later this year.