By James Mackenzie
ROME (Reuters) - Silvio Berlusconi launched a fierce attack on magistrates at a stormy political rally on Saturday, accusing them of trying to eliminate him politically but he pledged to keep supporting the fragile coalition of center-left Prime Minister Enrico Letta.
The leader of Italy's center right, whose appeal against a tax fraud conviction was rejected this week and who still faces trial on charges of paying for sex with a minor, repeated his longstanding accusations against prosecutors.
"Anyone who is not caught up by political factionalism can see clearly that there are politically motivated magistrates who are blinded by hate and prejudice towards me," he said.
His speech was frequently interrupted by whistles and jeers of "In jail! In jail!" by rival demonstrators as well as chants of "Silvio! Silvio!" by supporters of his center-right People of Freedom party.
Police intervened to keep rival groups apart and there was no sign of serious violence.
The rally added to the simmering tensions within Letta's uneasy coalition between traditional rivals on the left and right, which was formed after the inconclusive national election in February left no party able to govern alone.
However, Berlusconi said there was no prospect of him withdrawing support from Letta, who he said had promised to accept the center right's demand for billions of euros of tax cuts.
"They thought I could pull out and put the survival of the government at risk," he told a rally in the northern city of Brescia. "But as always they were mistaken. We intend to continue to support this government."
Berlusconi is not a minister in Letta's coalition, but plays a decisive behind-the-scenes role and could bring down the government if he were to withdraw support in parliament.
The 76-year-old media tycoon faces a new hearing on Monday in his trial on the charges of paying for sex with nightclub dancer Karima El Mahroug, alias "Ruby the Heartstealer" when she was under the age of 18.
In addition, Naples prosecutors called for him to face trial on separate charges of bribing a senator to defect to his party in order to help bring down the government of former center-left prime minister Romano Prodi, which fell in 2008.
Berlusconi's legal battles and his frequent attacks against magistrates have fuelled the unease which many on the center left feel about being forced to join forces with their main rival of the past two decades.
The parties are already at loggerheads over Berlusconi's demands to scrap an unpopular housing tax and calls by Cecile Kyenge, Italy's first black minister, for citizenship rights for children born in Italy of migrant parents.
Leading members of Letta's Democratic Party (PD) criticized the rally in Brescia and in particular, the decision by Interior Minister Angelino Alfano, from Berlusconi's People of Freedom, to attend a demonstration against state prosecutors.
"Alfano, the deputy prime minister, is feeding the tensions between the political forces which support the government," said center-left Deputy Economy Minister Stefano Fassina.
Letta himself made no direct comment on the rally at a special assembly of PD leaders on Saturday, saying only that his government respected "the independence of the magistrates always, whatever happens".
The PD, weakened by infighting after February's election, chose Guglielmo Epifani, former leader of the CGIL trade union as party secretary on Saturday.
Neither of the party's two most prominent leaders, Letta and the young mayor of Florence, Matteo Renzi, ran in the leadership ballot, with the real battle to lead the center-left likely to be delayed until a full party congress in October.
Meanwhile, Berlusconi, whose alliance has taken a lead in the opinion polls since the vote, has stepped up his battle against magistrates as the end of the Ruby trial approaches.
His own Canale 5 television station is due to broadcast an extended special on Sunday on the so-called "bunga bunga" parties at his luxurious residence near Milan that judges say was where the alleged sex offences took place.
He has always strenuously denied the accusations and said the parties were elegant and entirely respectable dinner parties attended by a wide variety of people.
(Editing by Alison Williams)