The car bomb exploded at 8.05am (0405 GMT) on Sunday, in front of the police station at Somar in south-east Mosul, said Captain Nidam Muhammad.
Of the five dead, three were policemen and two civilians, reported Abd Al-Qahhar Jumaa, Aljazeera's correspondent.
The driver of the car drove up at great speed. The police opened fire at him as it passed the first barricade, killing him immediately, police sources told Aljazeera.
The car, a Japanese four-wheel drive, then exploded as it rammed into a police car at the site. About 51 people were injured in the explosion, most of whom were transferred to al-Salam hospital, the correspondent said.
The Baghdad blast killed two
civilians and wounded two
A number of houses nearby and 12 cars, including five belonging to the police, were destroyed in the explosion, the correspondent added.
The explosion was so intense that it left a huge crater 2m deep, he said.
The explosion came just four days after 70 people were killed in a massive human bombing outside a rapid reaction police unit in the town of Baquba, northeast of Baghdad.
Last Monday, four people, including a child, were killed and five were wounded when a bomber blew a car up outside the main US military base in Mosul.
In the capital Baghdad, a roadside bomb exploded on Sunday as a convoy carrying foreign civilians drove past, killing two people and wounding two more in a central area of the city, police said.
The blast sent plumes of black smoke rising above Abu Nawas street, on the eastern banks of the Tigris River in central Baghdad.
Fawad Allah, an officer at Karradah police station, said a man and a woman were killed in the explosion and two other civilians were injured. The occupants of the foreign civilian vehicles were unhurt.
Bombings and shootings frequently target Iraq's fledgling US-backed police and security forces.