Leading opposition politicians in Italy have launched attacks on Mr Berlusconi after one of his close allies was convicted of Mafia collusion.
Marcello Dell'Utri will only serve his sentence if appeals fail
The verdict came a day after another Italian court rejected separate charges brought against the prime minister.
Centre-left politicians say the decision proves Mr Berlusconi and his allies are unfit to govern.
Marcello Dell'Utri, a founder-member of Mr Berlusconi's Forza Italia party, was sentenced to nine years in prison.
A close friend and political adviser to the prime minister, Dell'Utri has vowed to appeal against the verdict.
Describing Dell'Utri's conviction in a Palermo court as a "moral judgement" on Mr Berlusconi himself, centre-left leaders say the case has revealed a political class unworthy of government.
"Yesterday, Berlusconi was saved by the skin of his teeth, and today his devoted follower, Marcello Dell'Utri, is sentenced to nine years for Mafia association," said
opposition lawmaker Antonio Di Pietro.
Mr Di Pietro, a former prosecutor who made his name during the Clean Hands anti-corruption investigation of the 1990s, said that Mr Berlusconi should be removed from office by the calling of general elections, not scheduled until 2006.
The BBC's David Willey in Rome says Mr Berlusconi's celebrations have been cut short by the result of the Dell'Utri case, which he says is a serious political setback for the prime minister.
Mr Berlusconi had been accused of bribing judges in the 1980s to favour his business interests.
But he escaped conviction when the court in Milan invoked a statute of limitations, meaning the time had run out to sentence him.
The ruling implied that he was guilty of one count of bribing a judge in Rome in 1991, but could not be sentenced because of the time limit.
But Dell'Utri was not so fortunate.
The prosecution said he had acted as a link between organised crime in Sicily and Italy's business and political elite.
The senator's co-accused, Gaetano Cina, was sentenced to seven years' jail.
Mr Berlusconi had been called as a witness, but declined to give evidence as he was entitled to under Italian criminal procedure.
Evidence of Dell'Utri's complicity in money-laundering and contact with notorious Mafia bosses was given to the court by over 40 former Mafia members.
However, our correspondent says Dell'Utri will not go to jail.
Under Italy's slow-moving justice system he will not have to serve his sentence until this has been confirmed by two courts of appeal, which could take many more years.
He will also be allowed to continue to serve a senator until both appeal trials are concluded.