Several police officers are reportedly among the victims.
Several more police officers, as well as civilians, were wounded in the blast.
Hadhar lies some 90 kilometers south of Mosul, where a number of police officers were reported killed or wounded in a series of bombings and heavy clashes between insurgents and U.S. and Iraqi forces earlier in the day.
Several car bombs and roadside bombs were triggered in the city, while mortar shells were fired at police posts.
The United States has said it is pulling some 3,500 troops out of Mosul to reinforce security in Baghdad.