Berlusconi, 76, also lashed out at the "dictatorship of magistrates" responsible for his conviction and, while confirming he won't run for premier in spring elections, announced an undefined new movement to reform Italy's justice system.
Berlusconi delivered an at times breathless 50-minute speech in one of his Milan-area villas and took another 40 minutes of questions, a day after a Milan court sentenced him to four years in prison and barred him from public office for five years in a decade-old case involving the purchase of TV rights of U.S. films for his media empire.
The sentence isn't definitive until all appeals are exhausted, and Berlusconi's lawyers vowed to appeal. He remains free and is unlikely to serve jail time given his age and the possibility that the statute of limitations may expire before the two levels of appeals are completed.
But he nevertheless came out fighting Saturday, saying he had decided to end a self-imposed media silence since resigning from office last year, because he couldn't stay quiet anymore.
He announced what sounded like a political platform to undo many of Monti's reforms. And he warned that his People of Freedom party would be meeting in the coming days to decide whether to withdraw support from Monti's government and force early elections.
He said it would do so to end the "recessive spiral" that Monti's reforms had brought about.
The rambling speech got personal at times, such as when he denounced a now-famous smirk about him shared by German Chancellor Angela Merkel and France's then-President Nicolas Sarkozy, which he termed an "attempted assassination of my international credibility."
Berlusconi had wavered about his political future in the year since he was forced from office amid sex scandals and his inability to reassure financial markets that he could push through the economic reforms needed to ward off a debt crisis.
Monti was tapped to head a technical government, which has pushed through a series of tax hikes, structural reforms and austerity measures that, while hurting ordinary Italians, have significantly brought down Italy's borrowing rates.
On Saturday, Italians in several cities took to the streets for a "No Monti" day of protests.
Monti had initially said he wouldn't run in spring elections, but recently hinted that he could stay on for a second term under the right circumstances. Berlusconi had even come out and supported Monti after saying he wouldn't seek a political comeback.
But apparently things changed after the billionaire media mogul received the stiffest sentence among the four co-defendants convicted Friday in a scheme that involved inflating the price his Mediaset media empire paid for TV rights to U.S. movies and pocketing the difference.
Berlusconi called the conviction "absurd" and said if a country can't count on impartial judges, it ceases to be a democracy.
He blamed Merkel for many of Italy's woes, criticized Monti's fiscal reforms as contributing to Italy's recession and insisted his only error in the past was to not have secured a greater parliamentary majority in 2008 last elections.
Berlusconi's re-emergence on the political scene came a day before his beleaguered party heads into a regional election in Sicily seen as a test of its ability to pull itself together after Berlusconi's fall from grace last year and a series of local political corruption scandals that have soured Italians on their entire political class.
The Sicily vote is a harbinger of what Italy may witness in the spring as voters go to the polls for a general election amid recession and a political transition that has seen comic Beppe Grillo's populist Five-Star Movement threaten Italy's traditional center-right and center-left parties.
During his three stints as premier, Berlusconi had sought to reform Italy's notoriously slow and inefficient judiciary. In the absence of a wholesale reform, his forces in parliament passed several laws designed to help him and his colleagues in their legal woes, including passing immunity bills that temporarily halted trials against him.
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