Police in Milan also said that a letter bomb addressed to the Northern League, a government ally, exploded in a post office and slightly injured a postman. An anarchic group claimed responsibility.
Bruno Megale of Milan's anti-terror police said the group was protesting the government's crackdown on immigration. The same group planted a small device that was found half-exploded at Milan's Bocconi university last year.
Local balloting across Italy this weekend is seen as a test of popularity for Berlusconi.