Italian police were investigating the causes of explosions in central Rome today which are not thought to have had a specific target, a senior official said. Nobody was injured in the blasts.

A rudimentary petrol bomb damaged a car when it produced two explosions in the car park of a post office near the ancient Circus Maximus racetrack, police chief Massimo Improta said.

"We do not believe there was a plan to strike anyone in particular with the device, it is definitely an act of protest," Improta told SkyTG television, adding sniffer dogs had been deployed and the situation was now under control.

Forensic police at the scene found the remains of what appeared to be a plastic bottle which could have contained at most one litre of petrol, and a kitchen timer, Improta added.

"The explosion could have struck without causing excessive damage," he said. He declined to comment when asked by a reporter whether anarchist activists could be behind the attack, which took place across the street from a fire station.

Witness Giulia Scalondro, who works in a nearby cafe, said she heard one explosion followed by a stronger one.

"We were frightened and ran out of the cafe. We saw high flames and a thick dark cloud," Scalondro said.

Italy is hosting annual summits of the Group of Seven industrialised nations, culminating with a leaders' meeting in Sicily at the end of May. The national security alert scale is already set at 2, the highest possible in the absence of an ongoing attack.