Police in Rome say an explosion outside a post office appears to have been caused by a rudimentary explosive device placed between two parked cars.
The blast Friday morning during business hours in the parking area near a side entrance to the post office caused no injuries but damaged a vehicle.
The post office serves the residential Testaccio neighborhood and is near the foot of the Aventine Hill.
Similar explosions in the past in Italian cities have been frequently blamed on anarchists.
There was no immediate claim of responsibility.