Forlì, August 30 - Florence Mayor and rising star in the center left Matteo Renzi slammed ex-premier Silvio Berlusconi for not stepping down from his Senate seat after a definitive conviction for tax fraud from the supreme Court of Cassation. "In any civilized country, a leader who's definitively convicted leaves on his own, without waiting to be kicked out," said Renzi at a rally for the Democratic Party (PD) in Forlì. A Senate committee is set to begin proceedings September 9 to determine whether to strip Berlusconi of his seat in the upper house as part of an anti-corruption law that kicked in after his August 1 conviction for fraud on film rights at his Mediaset media empire. Berlusconi has appealed to the European Court of Human Rights and Italy's Constitutional Court. The three-time premier dismissed the explanation of the sentence, issued earlier Thursday, as a "collage" of previous sentences from magistrates he has always seen as left-wing. His party has threatened to pull its support from the fragile executive and force snap elections if he loses his Senate seat. Renzi, a charismatic 38-year-old seen as a likely candidate for premier, was making a highly anticipated return to the political soap box after summer holidays.