By John Irish and Adrian Croft
PARIS (Reuters) - Embattled French presidential candidate Francois Fillon delivered a defiant speech to thousands of supporters in central Paris on Sunday, but made no pledge to fight on as pressure from his conservative party mounted on him to step aside.
Once the frontrunner in the presidential race, 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.
Party leaders prepared for a meeting on Monday to discuss the crisis ahead of a March 17 deadline when all candidates must be formally endorsed by at least 500 elected officials.
A senior politician from his party, The Republicans, had earlier said that several party heavyweights were about to issue a statement calling for former prime minister Alain Juppe, who lost to Fillon in November for the party ticket, to replace him.
As thousands of tricolored flag-waving supporters braved pouring rain and chanted for him to stay, Fillon appeared to put the ball back into his party's court.
"I will continue to tell my political friends that the choice is down to them, and also not down to them," he said in a half-hour speech laced with attacks on the outgoing Socialist government, far-right National Front leader Marine Le Pen and the favorite, independent Emmanuel Macron.
"Because the choice is yours, those of your votes and through those votes your hopes. I am sure that it will be all of France's choice if we are able to gather ourselves in an ultimate push."
However he made no promises to battle on until the end.
"I am attacked from all sides and with all consciousness I must listen to you, listen to this massive crowd that pushes me forward, but I must also ask myself about those who doubt me and jump ship," he said.
Polls continue to show Fillon would fail to make the second round of the election. A Kantar Sofres-OnePoint survey published on Sunday showed Fillon down to 17 percent, well behind Macron and Le Pen in voting intentions for the April 23 first round.
But it showed that Juppe would reach a May 7 run-off against Le Pen if he replaced the man who defeated him in November's party primaries.
After a string of resignations among advisers and backers, the 63-year-old Fillon had been banking on a big turnout at the rally, on a square looking across the river Seine to the Eiffel Tower, to show his detractors that he remains their best hope to win the presidency.
But others in his party were already looking for a way to oust him.
Christian Estrosi, a close ally of former President Nicolas Sarkozy, told BFM TV that he and other party heavyweights would make a statement in the coming hours to offer an alternative.
"The easiest thing obviously ... is the person who came second in the primaries and that quite simply is Alain Juppe."
Jean Leonetti, a pro-Juppe lawmaker in the party said Sarkozy and Juppe had spoken on Saturday night and Juppe had probably outlined his conditions for replacing Fillon.
Calling for Juppe to take over, Jean-Christophe Lagarde, head of the center-right UDI party, which has an alliance with The Republicans, said Fillon would lead to "certain failure."
"In the Olympics when the gold medal winner is disqualified then it's the silver medal holder that takes over," Lagarde said on Europe 1 radio.
Fillon, who has denied any wrongdoing and complained of judicial and media bias that amounted to a "political assassination", will appear on France 2 television's Sunday evening news.
A member of his staff said it would be an opportunity to "speak to the French."
Sunday's rally was intended to show he still has support among the grassroots.
"I think the campaign against Fillon is absolutely scandalous," said Annette Dore, 62, a former scientific publisher from Paris. "Why reproach him when he was about to win the election? It's so dishonest," she said.
(Additional reporting by Simon Carraud, Sudip Kar-Gupta, Emmanuel Jarry and Andrew Callus; Editing by Ros Russell)