The press secretary for the governor of Helmand Province, Dawood Ahmadi, said the incident occurred at 10:00 am (05:30 GMT) near a police station in the province's capital, Lashkar Gah. Nine police officers died at the scene, and another three died in hospital. Twenty-three other people, including several civilians, were injured.
Most of the victims of the attack were counternarcotics police who were preparing for an opium raid in the area. Helmand Province is the world's largest heroin-producing region.
The radical Taliban movement has claimed responsibility for the bombing.