Silvio Berlusconi suffered arguably the heaviest blow of his political career on Wednesday when the upper house of parliament voted to oust him following a conviction for tax fraud.

A hostile front of the centre-left and anti-establishment Five Star Movement (M5S) voted against the former prime minister, who pulled his Forza Italia party from Enrico Letta's governing coalition and into opposition on Tuesday.

Berlusconi was not present for the Senate vote. But shortly before the chamber approved his expulsion he gave a defiant address to supporters outside his residence in central Rome, declaring a "day of mourning for democracy" and promising that he would remain on the political scene.

"Today they are toasting because they can take an adversary, they say a friend, in front of the executioner's squad," Berlusconi said. "It is the day they have been waiting for for 20 years."

He pledged to continue his role as a political leader, citing other figures not in Parliament, namely the founder of the M5S, Beppe Grillo, and Matteo Renzi of the Democratic Party, tipped by many as a future premier candidate.

"Also, from outside the Parliament, we can continue to fight for our liberty," he said.

Berlusconi, who resigned as PM in late 2011 amid concerns over Italy's growing financial instability, received his first definitive conviction in 20 years of legal battles on 1 August. He was sentenced to four years in prison, commuted to one year of community service.

The debate over the parliamentary ramifications of the conviction has dominated the national political scene for the past four months. The 77-year-old media magnate has issued alternate pleas and threats in an attempt to avoid being stripped of his seat under a law passed last year – with the support of his then party, the Freedom People – which stipulates that MPs convicted of serious criminal offences must be ineligible for parliament.

Berlusconi kept up the battle until the last minute, claiming on Monday to have new evidence that he said would exonerate him, and begging his fellow senators to put off the vote until the documents had been examined.

He insists the conviction is another sign of his continuing persecution by leftwing judges. He has indicated that Giorgio Napolitano, the Italian president, should pardon him without him having to ask – an idea that drew a terse response from the former communist head of state.

He is expected to begin serving his sentence next year for the tax fraud conviction, which related to a complex system of illegally inflated invoices at his Mediaset television empire. But this is not the end of his legal woes. Among other matters, he has been ordered to stand trial on charges of bribing a senator in an attempt to bring down Romano Prodi's government, and is appealing against a first-grade conviction handed down in June for having sex with an underage girl and abusing his office to cover it up. He denies the allegations in both cases.

Despite his expulsion, Berlusconi will by no means disappear from the political scene. His future role has been compared to that of Beppe Grillo, the M5S's figurehead who himself has not been elected.

The expulsion vote will heighten the tensions that have plagued the Letta government from its inception this year, even if, with a breakaway centre-right group that remains loyal to the coalition, it has a reasonably secure if small majority.

With their leader kicked out of the senate, Forza Italia MPs could prove highly disruptive in opposition and could stymie the kind of institutional reforms Letta says he wants to pass.