Rome, February 28 - Italian ex-premier Silvio Berlusconi is being probed in Naples for suspected corruption and illegal party funding, judicial sources said Thursday. The case regards money allegedly paid to Senator Sergio De Gregorio, who defected from the centre left during Romano Prodi's 2006-2008 government and eventually joined Berlusconi's party. The switch of allegiance helped undermine Prodi's majority in the upper house and the administration collapsed less than two years after taking power. De Gregorio allegedly received three million euros. A safety deposit box in Berlusconi's name was seized by Italian police on Thursday and the three-time Italian premier was summoned to appear before Naples prosecutors on March 5. The 76-year-old media magnate's People of Freedom (PdL) party said the move signalled that some prosecutors have resumed their alleged judicial "persecution" of Berlusconi after a truce for Sunday-Monday's general election. Berlusconi's centre right came second in the election, but managed to prevent Pier Luigi Bersani's centre-left alliance gaining a working majority in the Senate, which his supporters have hailed as a success as the alliance trailed by double figures in the polls at the start of the election campaign. "The comments on our election success have not yet finished in the newspapers and the attacks by the magistrates against Silvio Berlusconi have already started again," said Angelino Alfano, the PdL secretary. "We announce that we are going to organise a big demonstration in the streets to defend the sovereignty of the PdL and of Italian democracy. "All this is taking place while the country is going through a highly delicate phase of institutional transition in which Berlusconi will be called to exercise great influence as the leader of the PdL. "This circumstance makes it clear that the magistrates' actions are political". Berlusconi is currently on trial in Milan over accusations he paid for sex with a young Moroccan dancer, Karima El Mahroug, better known as Ruby the Heart-stealer, before she was 18 during alleged bunga bunga sex parties at his home. Both the ex-premier and the woman deny ever having sex. She said money she received from Berlusconi was given as part of a gift. Berlusconi is also accused of abuse of office for allegedly having used his influence when he was premier to spring El Mahroug from a Milan police station to hush up the affair after an unrelated theft claim. Berlusconi is appealing against a one-year conviction he was handed last year for tax fraud on film rights for his Mediaset TV group. The three-time premier has also been indicted over accusations he was involved in the publication of an illegally obtained wiretap. In the ongoing and several other previous trials, Berlusconi has always denied wrongdoing, claiming he is the victim of a minority group of allegedly left-wing prosecutors and judges who he says are persecuting him for political reasons. Berlusconi has been tried some 30 times but has only been convicted three times - verdicts that were either timed out or overturned on appeal - prior to the Mediaset fraud verdict. "Now that the elections are over and the voters have once again reiterated their confidence in Berlusconi, the use of the justice system for political ends has resumed in some prosecutors' offices," said PdL MP Maurizio Lupi. Valter Lavitola, a former associate of Berlusconi's, is also being probed in relation to De Gregorio's switch. Lavitola and Berlusconi are being probed in a separate case regarding allegations they induced a witness to lie to investigators about the alleged sex parties at the former premier's home. Earlier on Thursday, prosecutors in the northern city of Reggio Emilia said they had opened a probe into a letter Berlusconi sent to millions of Italians about an election pledge to pay back the IMU property-tax. Berlusconi was accused of attempting to buy votes with the pledge by rival politicians, who complained the letter was deceptive as it looked like an official document informing people about how to obtain a rebate, rather than an election pamphlet. The Reggio Emilia probe was opened after a member of the public lodged a complaint. Rome prosecutors are also investigating after a similar petition.