A rudimentary explosive device detonated between parked vehicles outside a post office in Rome, causing no injuries but damaging a car.

A preliminary investigation indicated that the blast was likely to have been “a demonstrative act, showing that it could be done”, rather than being devised to cause major damage, Rome police official Massimo Improta told reporters on Friday.

Similar explosions in Rome and other Italian cities in recent years have frequently been blamed on or claimed by anarchists. There was no immediate claim of responsibility for the latest blast.

The post office, an imposing, 1930s fascist-era building that is often studied by architecture students, remained open throughout the incident.

The small blast occurred in a parking area outside a side entrance where people go to pick up mail the post office could not deliver because no one was home.

Improta said a trigger, set off by a timer, initiated the explosion, which then ignited gasoline inside a plastic container that was part of the device, a noisy sequence leading some people at first to think there were two explosions.

“The assembling of the materials wasn’t haphazard, but it was done by an incompetent person, or people,” Improta said. “We do not think the intention was to strike anyone.”

The post office serves the residential Testaccio neighbourhood and is near the foot of the Aventine hill, the southernmost of Rome’s seven hills.