We are going to try and hold the Nato summit in a way that there will be no incidents

Silvio Berlusconi

The blast took place early in the morning on the day the Mr Berlusconi was due to visit the city.

Mr Berlusconi has also admitted that he and Interior Minister Claudio Scajola had received threatening letters and envelopes containing bullets, adding "it is routine now".

The blast caused extensive damage to the building and shattered windows up to 200m away. Two policemen were reportedly injured.

Security concerns following the violence surrounding last month's G8 summit in Genoa have forced Italy to reconsider holding future summits in the country.

UN summit in doubt

It seems likely that next month's Nato summit will go ahead in Naples as planned, despite objections from the city's mayor.

But Mr Berlusconi has suggested the UN's Food and Agriculture (FAO) summit on global hunger in November might be moved to a location in Africa.

Berlusconi: Italian Government is under attack

He said it would be irresponsible not to be concerned about holding another summit after Genoa.

One protester was killed and hundreds injured and arrested during the violence in Genoa, sparking international outrage at the Italian police's handling of the event.

Italy's chief of police on Wednesday admitted that officers used excessive force in trying to bring demonstrators under control.

"We are going to try and hold the Nato summit in a way that there will be no incidents," Mr Berlusconi said.

Terrorism fears

Thursday's bomb has raised fears of a prolonged campaign of violence in Italy.

"My feeling is that this is an attack that wants to send a signal to someone," Venice Mayor Paolo Costa said.

Mr Berlusconi said Italian institutions had become the target of "an international movement that was not born in Italy but appears to have as its aim a fight against the Italian Government".

The presidents of the Senate and the House of Representatives issued an "urgent appeal to all political movements and public opinion to unite around the democratic institutions in order to isolate all forms of violence and terrorism".

In the 1970s Italy was the scene of a terrorist campaign led by the far-left Red Brigades, or Brigate Rosse.