The Italian premier Mr Silvio Berlusconi, government ministers and police officials have spoken of their concern after a bomb went off in Rome.
The device damaged parked cars and shattered windows near the Interior Ministry.
Nobody was injured but the explosion has shaken the country which is on high alert against terrorism. There has been no immediate claim of responsibility for the blast.
Mr Berlusconi said: "Undervaluing it would be a mistake," adding that it was "a worrisome signal".
Interior Minister Mr Claudio Scajola described it as "a very serious act carried out against the symbol of the state's security".
Colonel Gianfranco Cavallo, of the Carabinieri paramilitary police, said the attack was "an act of show" rather than an attempt to harm anyone".
He said: "The street is virtually deserted at that hour, there is no traffic, and even the explosive device was not that powerful. If they wanted to cause serious damage, they would have placed the bomb in front of a door."
The device was planted on a moped around the corner from the Interior Ministry, headquarters of Italy's national police and security services.
Nearby windows were shattered, rubbish bins and parked vehicles damaged and the street was dusted with broken glass. The bombing provoked expressions of concern from the president.
Bomb experts have been combing the area for fragments of the device and security officials are reviewing tapes from surveillance cameras outside the ministry building and interviewing witnesses.
PA