The incident occurred early Monday as the guardsmen queued up outside the station to collect their salary at the police station near Ramadi, some 150 km west of Baghdad.
The bombing is latest instance of stepped up insurgent activity after Coalition forces razed the rebel stronghold of Fallujah earlier this month.
US forces are now concentrating on Mosul, where at least 50 people, including seven US Marines, have reportedly been killed over the past week.
On Sunday, rebels stormed a police station in Samarra and escaped after looting the armory.