A police lieutenant says 16 police officers are dead, as well as two civilians, including a seven-year-old girl.
The casualties from Sunday's explosion in Khalidiyah were taken to a hospital in Ramadi, 110km west of the capital, said hospital sources.
A Ramadi general hospital official said five of the injured are in a serious condition.
A US officer says there is "strong evidence" that the attack was a "suicide bombing".
Black smoke billowed over the police station in Khalidiyah, 60 km west of Baghdad, as ambulances rushed to the scene and evacuated casualties.
The US military said there were no casualties among occupation troops.
No further details of the attack were immediately known.
Instability and lawlessness continue to grip Iraq, eight months after US and British troops swept into Baghdad and ousted leader Saddam Hussein.
The Iraqi resistance has regularly targeted policemen whom they consider agents of the US-led occupation.
At least 18 Iraqis were killed and more than 30 wounded by almost simultaneous bombings of two police stations north of Baghdad late last month.