Police and paramedics were dealing with a crash on the A35 Upton Bypass shortly after 03:20 GMT when a bus collided with the emergency vehicles.
Two people suffered minor injuries. Dorset Police said both the police car and ambulance were stationary at the time of the collision.
The road remained closed westbound at the A350 junction until 10:00.
Traffic had been diverted via the A31 Roundhouse Roundabout and the A31 west to Bere Regis.