By Catherine Hornby and Antonella Ciancio

ROME/MILAN (Reuters) - Silvio Berlusconi on Saturday condemned outgoing Prime Minister Mario Monti's plan to lead a centrist alliance in Italy's election in February, accusing him of a plot to favor the left, but centrist leaders denied any secret accord.

Monti, who replaced Berlusconi as prime minister last year as Italy scrambled to avert a financial crisis, said on Friday he wanted to unite a broad coalition of factions around a reform agenda aimed at easing the country's economic woes.

Monti ended weeks of speculation when he confirmed his bid for a second term, pitting him against the centre-left Democratic Party (PD) and Berlusconi's centre-right People of Freedom (PDL) party in a three-way contest.

Speaking to reporters at Milan Central railway station, Berlusconi said Monti wanted to help the left secure power after the February 24-25 election so he could continue his austerity agenda of tax hikes and spending cuts.

"This grouping has been formed to favor the left - also the harmony with the left's program they have celebrated heads in this direction," he said, after earlier describing Monti as "the spare wheel" of the PD in an interview with Vista TV.

The 76-year-old billionaire, who caught the train from Rome with his new 27-year-old partner, Francesca Pascale, said he did not believe Italian voters would "fall into the trap" which he said was aimed at stealing votes from the centre right.

But Pier Ferdinando Casini, head of Italy's oldest and largest centrist party, the UDC, which is cooperating with Monti, strongly denied the accusations.

"Our initiative was not born with the support of the PD. It has not been started with a predetermined alliance ... until election day, what's important is aiming for the majority," Casini said at a news conference on Saturday.

"PHASE OF RESPONSIBILITY"

Opinion polls suggest the PD, under Pier Luigi Bersani, will win a comfortable lower house majority but may have to strike a deal with centrist forces in the Senate, where the centre left has struggled to gain control in past elections.

The PD, which has pledged to maintain Monti's broad reform course while putting more emphasis on jobs and growth, has urged the 69-year old technocrat to clarify the approach the centrist forces will take towards the left.

"Will they present themselves as alternatives, as rivals, or as open to an alliance?" Bersani asked on SkyTG24 television on Friday, saying that the centre left would be open to discuss an accord when Monti's position is clear.

Monti, a former European Commissioner, is a favorite with international investors, the Catholic Church and the business establishment, and has been widely credited with restoring Italy's credibility after the scandal-plagued Berlusconi years.

"For the first time an atmosphere is forming that points towards the future for a Europe that needs Italy and a country that wants to change deeply," centrist leader Casini said on Saturday.

"From yesterday we are putting behind us the empty electoral promises, populism, demagoguery, fake assurances, a phase of responsibility is beginning," he said.

The PD has so far maintained a tone of polite respect for Monti, in contrast to Berlusconi's attacks on his "Germano-centric" austerity policies, which he blames for deepening a severe recession and fuelling record unemployment.

The media tycoon also said on Saturday he was disappointed that Monti had made a bid for a second term because the economics professor had told him he would not use the exposure gained as an unelected technocrat for future political motives.

Berlusconi added that if the centre right won the election he would launch an investigation into Monti's ascent to power.

(Editing by Alison Williams)