* PD's infighting compounds difficulties for new PM Letta
* Former head of CGIL union sole candidate as party secretary
* Tensions over planned Berlusconi demonstration against judges
By Massimiliano Di Giorgio
ROME, May 11 (Reuters) - Italy's centre-left Democratic Party (PD), reeling from infighting that forced the resignation of Pier Luigi Bersani last month, chose former trade union boss Guglielmo Epifani as party secretary on Saturday.
The party's two most prominent leaders, Prime Minister Enrico Letta and the young mayor of Florence, Matteo Renzi, both sat out the internal party contest, with the real battle to lead the centre-left likely to be delayed until after the summer.
The PD's weakness has compounded problems confronting Letta as he tries to hold a fragile coalition together while facing deep recession, youth unemployment of almost 40 percent and public bitterness reflected in the success of the anti-establishment 5-Star Movement.
Epifani, a former head of Italy's biggest union the CGIL, was the sole candidate at a special assembly in Rome but he was widely seen as a compromise stand-in until a full party congress in October.
"The Democratic Party has been going through an extremely difficult period," Epifani told the assembly. "We cannot hide it from ourselves, we risk hitting the bottom."
The PD threw away a strong lead in polls ahead of February's election, and the inconclusive result forced it into an unlikely coalition with arch-rival Silvio Berlusconi.
Divided between a dozen competing factions, the party has been effectively leaderless since Bersani resigned following a humiliating party mutiny during the process of voting for a president of the republic in April.
Opinion polls show Berlusconi's centre-right alliance now well ahead of the centre-left.
With tensions between the PD and Berlusconi's People of Freedom (PDL) party already threatening to destabilise the government, centre-left party barons were desperate to avoid a damaging battle over the leadership.
The two are already at loggerheads over centre-right 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.
BERLUSCONI PROTEST PLANNED
The PD had also criticised the PDL's plan to hold a demonstration on Saturday against what it says is a campaign by politically biased magistrates against Berlusconi, and reacted furiously after Interior Minister Angelino Alfano, the deputy head of the government, announced on Twitter that he would attend.
"Alfano ... is feeding the tensions between the political forces which support the government," said centre-left Deputy Economy Minister Stefano Fassina.
The 76-year-old media tycoon's appeal against a four-year sentence for tax fraud was rejected by a Milan court this week. He also faces a new hearing on Monday in his trial on charges of paying for sex with nightclub dancer Karima El Mahroug, alias "Ruby the Heartstealer" when she was a minor.
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 centre-left prime minister Romano Prodi, which fell in 2008.
Berlusconi's 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 which 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. (Writing by James Mackenzie; Editing by Sonya Hepinstall)
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