(Corrects to add dropped word 'former' in first paragraph)
* Sarkozy ally Estrosi says top politicians preparing initiative
* Estrosi, centrist UDI chief Lagarde say Juppe best option
* Fillon party leaders to meet Monday for crisis talks
* Mass rally planned on Sunday in Paris to support Fillon
* Election graphic tmsnrt.rs/2jLwO20
By John Irish and Simon Carraud
PARIS, March 5 (Reuters) - A senior politician from embattled French presidential candidate Francois Fillon's conservative camp said on Sunday that several party heavyweights were about to issue a statement calling for former prime minister Alain Juppe to replace him.
Once the frontrunner, Fillon is mired in a scandal over his wife's pay, and his campaign has been in serious trouble since he learned last week that he could be placed under formal investigation for misuse of public funds.
He is under growing pressure as party leaders prepare a crisis meeting for Monday to discuss the situation ahead of a March 17 deadline when all presidential candidates must be formally endorsed by at least 500 elected officials.
After a string of resignations among advisers and backers, the 63-year old former conservative prime minister is banking on a rally of supporters in Paris on Sunday to show his detractors that he remains their best hope to win the presidency.
"In the coming hours, we will propose an initiative," Christian Estrosi, a close ally of former President Nicolas Sarkozy, told BFM TV, adding that it would be before Monday morning.
"We do not have the time to debate who has the most talent. I don't think any of the forty-somethings in our political movement, who have talent, can take on the role to bring us together."
He said the initiative would come in the form of a statement led by himself and other heavyweights in the Republicans party including Xavier Bertrand and Valerie Pecresse.
"The easiest thing obviously ... is the person who came second in the primaries and that quite simply is Alain Juppe," he said.
Fillon, who was set to attend a rally of thousands of supporters in central Paris, announced on Twitter that he would appear on France 2 television's Sunday evening news.
A member of his staff said he wanted to "speak to the French."
Jean-Christophe Lagarde, the head of the centre-right UDI party, which has an alliance with the Republicans, warned on Europe 1 radio that sticking with Fillon would lead to "certain failure" and called for Juppe to step in.
"In the Olympics when the gold medal winner is disqualified then it's the silver medal holder that takes over," Lagarde said.
Opinion polls continue to show Fillon would fail to make the second round of the April/May election. Instead, centrist Emmanuel Macron is consolidating his position as favourite to win a second-round head-to-head against far-right National Front candidate Marine Le Pen.
Fillon has denied any wrongdoing and complained of judicial and media bias that amounted to a "political assassination".
His attack on the judiciary in particular has caused unease within his ranks and Sunday's demonstration has worried some on the right that it will be hijacked by hardline conservative movements.
His backers are hoping to get at least 45,000 people at Sunday's rally to show he still carries favour among grassroots supporters. Despite the pressure mounting, Fillon remained defiant.
"We shall show the strength of popular will," he said in a post on Twitter ahead of the rally. "Hold firm! France deserves our courage!"
However, an Ifop poll of 1,002 people published on Saturday showed that more than 70 percent of French voters want him to drop out. Support from his camp has also fallen to 53 percent from 70 percent two weeks ago.
The same survey also suggested Juppe, who lost to Fillon in the November party primary, was the best placed to step in. A poll on Friday showed that Juppe would win the April 23 first round, although the current mayor of Bordeaux has until now ruled out a comeback.
Additional reporting by Adrian Croft; Editing by Dominic Evans