Italian officials have ordered an investigation into Facebook groups called Let's Kill Berlusconi, saying they incite hatred and might lead to a real rather than virtual attack on the prime minister.
There are at least three sites with the Italian title of Uccidiamo Berlusconi, with a total of some 16,540 members.
Each has a picture of Silvio Berlusconi in a red circle with a diagonal line across his face, on which is written: Berlusconi Get Lost.
These groups foment hate for Berlusconi, they call for his murder ... I expect magistrates to do their duty, said Justice Minister Angelino Alfano.
The media said police were considering blocking the first of the sites, which started late last year and whose numbers have grown in recent months as Berlusconi's legal woes and private life put him at the center of heated controversy.
Earlier this month, Italy's top court stripped Berlusconi of his immunity from prosecution, paving the way for the resumption of corruption trials against the conservative media mogul, a move his critics saw as a major victory.
But even opposition leaders, such as Democratic Party head Dario Franceschini, joined the center-right government in saying that Facebook, which is based in Palo Alto, California, should black out the site itself before police have to do it.
This is demented, Franceschini said.
Several hundred new members joined the Facebook groups on Thursday morning alone. Some of those who posted comments on Thursday said they joined after reading news reports, but rejected suggestions that they would actually harm Berlusconi.
They said the site was a provocation and intended to be a place where people could express their opposition to Berlusconi's government and his policies.
It is not the first time Facebook groups have come under scrutiny by Italian officials.
In January, leading Italian anti-Mafia activists expressed alarm that Facebook pages dedicated to the praise of some of Cosa Nostra's most notorious and bloody bosses had attracted hundreds of young admirers.
At that time, magistrates said those who joined those groups could in principle be investigated on suspicion of aiding and abetting organized crime.
(Editing by Andrew Roche)