The blast from a gas cylinder, hidden in a blue shopping bag, smashed windows but nobody was hurt, Ansa news agency reported.
The attack has not been claimed by any group, although police suggest it might have been the act of anti-capitalist or anarchist militants.
The target is believed to be the so-called ‘surveillance court’, which ensures sentences are carried out and rules on parole and bail.
The attack follows a bomb explosion outside a French church in Rome last Friday, which coincided with a visit to the city by France’s President, Francois Hollande.
The night-time explosion damaged a few cars and smashed windows on a small street where the church of Saint-Yves-des-Bretons is located.