* Renzi lost centre-left primaries to Bersani
* Says ready to challenge Berlusconi in election
* Florence mayor could be game-changer
ROME, April 4 (Reuters) - Florence mayor Matteo Renzi launched a clear challenge to Italian centre-left leader Pier Luigi Bersani on Thursday, calling for a government coalition with former premier Silvio Berlusconi or new elections in June.
Bersani won a majority in the lower house but not the Senate in February elections, leaving his centre-left group unable to govern alone. He failed last week in efforts to forge a viable majority in parliament after his overtures to the anti-establishment 5-Star Movement were rebuffed.
Renzi, 38, who lost to Bersani last December in a vote to be the centre-left election candidate, gave interviews to several Italian newspapers saying he was ready to be a candidate in new leadership primaries.
Renzi had previously hesitated to challenge Bersani, a former communist politician who threw away a 10-point opinion poll lead in the Feb. 24-25 election which left Italy in political deadlock.
But in recent days he has become increasingly outspoken in attacking Bersani's line that a "grand coalition" alliance with scandal-plagued centre-right leader Berlusconi is unthinkable.
Another option to emerge from the impasse is a technocrat government sponsored by President Giorgio Napolitano, but this is opposed by both centre left and centre right.
"We cannot stop here waiting for Bersani to get support ... It is ridiculous to stick with a frozen task. We must do something: a government formed by the president, a grand coalition, or we must return to vote," Renzi told La Repubblica daily.
The Florence mayor, who had been widely expected by analysts to make his move after Bersani's failure to secure a workable majority, rejected suggestions party primaries could not be organised by June, saying they could be held within two months.
Berlusconi's centre-right People of Freedom party (PDL) has repeatedly called for Bersani to join them in a coalition or go back to the ballot box in June, although analysts say there is a diminishing chance that an election can be held by then.
A new election held under the current electoral law would run a high risk of producing another deadlock but there is thought to be no prospect of changing it before a June election.
BERLUSCONI SUPPORT GROWS
The law gives a big bonus in the lower chamber, even to a party that wins by a tiny majority, as the centre left did in February, but requires a new government to win a confidence vote also in the Senate where majorities are allocated regionally.
A new vote would very likely produce another winner in the lower house who lacks a majority in the Senate.
Bersani says a snap election would be a disaster for Italy, saying it needs a solid government to tackle a deep recession.
Support for Berlusconi has been steadily growing in post-election opinion polls, which show the centre-right overtaking the centre left.
Renzi, whose market-friendly views are seen as appealing to centre-right voters, said he could pose the most credible challenge to Berlusconi, who is awaiting verdicts in two trials, one over allegations he paid for sex with a minor.
"Berlusconi wants a vote in June precisely in order to not give space to me. We can challenge him. If I run, he will be in trouble," Renzi told La Repubblica.
There was no immediate reaction from the leadership of Bersani's Democratic Party (PD) which fears any alliance with Berlusconi would split the centre left.
Renzi is opposed by the left wing of Bersani's group, including powerful trade unions. But the Florence mayor, a more dynamic campaigner than Bersani, is seen as a potential game-changer who could win back votes from the 5-star Movement of former comic Beppe Grillo, as well as the centre right.
His primary campaign last year promised to get rid of the political old guard, a battle cry which chimes with the demands of many supporters of the 5-Star Movement.
Napolitano this week appointed two committees of "wise men" to draw up a plan of reforms that could win the support of all parties. It is not due to report back until the end of next week.
Little progress in ending the crisis is expected until a vote starting on April 18 to elect a successor to Napolitano, whose mandate runs out in May and who is not allowed to call a parliamentary election in the final months of his presidency.