ROME (Reuters) - Italian center-right leaders increased pressure on Prime Minister Mario Monti on Sunday to run for a second term in an impending election as the leader of an alliance of "moderates".

"These could be one-off circumstances," said Angelino Alfano, secretary of the center-right People of Freedom party (PDL) of former premier Silvio Berlusconi. "We are moderates who want to unite to prevent a win by the left."

His speech at a Rome center-right conference appealed to supporters to back Monti as the best way to continue reforms, meant to defuse a debt crisis dangerous to the euro zone, and avert a win by the center-left Democratic Party (PD).

The PD now commands the largest chunk of voter support at 31 percent, compared to 16.5 percent for the PDL, according to pollsters SWG.

But support for Monti is far from universal in the highly fragmented center-right, which includes his most entrenched critics, the Northern League, who were necessary coalition partners in the last center-right government.

Monti's government of non-political technocrats had been supported by both Berlusconi's PDL and the PD. But the PDL withdrew its support 10 days ago, prompting Monti to announce his resignation and bring forward elections to a likely date in February.

But shortly after his party withdrew support, Berlusconi announced he would pull out as an election candidate if Monti were to run, in an apparent about-face.

In an interview on Sunday, Berlusconi repeated his pledge to withdraw if Monti agreed to run as the head of a center-right alliance, saying that above all he wanted to ensure the left did not win.

"We are waiting for a response from Professor Monti," Berlusconi, a media tycoon, said in the interview on one of his television channels.

But he went on to criticize policies associated with Monti's government, said he would cancel an unpopular property tax and steer Monti away from what he called "German" austerity policies.

The tone of the interview, in which Berlusconi announced his engagement to a woman 48 years his junior, indicated his own election campaign was not slowing. It devoted much time to criticism of trials against him and to the highlights of his political career.

MONTI SUPPORT

According to a poll published on Sunday by the Milan daily Corriere della Sera, a Monti candidacy has more support among PD than PDL voters, even though the center-left has repeatedly said Monti should not run as a candidate.

The poll indicated 44 percent of PD voters thought a Monti run would be positive, against 50 percent who thought it would be a bad thing. This compared to 19 percent support for the technocrat prime minister among voters for Berlusconi's party, against 78 percent who thought a Monti run would be negative.

The PD has expressed support for Monti continuing in some role after the election, possibly as president, as the current president, Giorgio Napolitano, is due to step down in May.

There is another possibility: that Monti could run at the head of an alliance of centrists and the business community now led by the president of carmaker Ferrari, Luca di Montezemolo.

Monti would significantly broaden support for the centrist group if he ran as its leader, increasing its vote to 15.1 percent from 9.3 percent, according to pollsters SWG.

Cesare Geronzi, the former chairman of Assicurazioni Generali, urged Monti on Sunday to announce his decision.

"I would rate him better if he shook off this reserve about his candidacy. The country is dying of uncertainty," Geronzi said in a television interview.

Napolitano, who met with Monti earlier on Sunday, said that the premier would announce his decision himself when questioned about it by reporters, but did not indicate when that would be.

(Reporting by Naomi O'Leary; Editing by Rosalind Russell)