PARIS (AFP) - French conservative candidate Francois Fillon told supporters on Saturday (March 4) their opponents were trying to "intimidate" them as he battles to keep his presidential bid afloat.

Marking his 63rd birthday, Fillon was trying to turn the page on a nightmare week that saw scores of defections from his camp after he admitted he is to face criminal charges over allegations he gave his wife and children fake parliamentary jobs.

With some members of his own rightwing Republicans party urging him to drop out of the campaign, Fillon told 1,500 supporters he had had "better" birthdays.

Without referring directly to his legal woes, he said at a campaign event north of Paris: "They are trying to intimidate you. They are attacking me. But through me, what they are trying to kill is a desire for change."

Fillon has denied wrongdoing and has claimed that the fake jobs charges are politically motivated, even hinting he believes the ruling Socialist government is behind the investigation.

As he was giving his speech, his Republicans party announced in a statement that its political decision-making body would meet Monday evening - a day earlier than planned - "to evaluate the situation".

Fillon will attempt to regain the initiative by holding a major rally on Sunday near the Eiffel Tower.

The danger for the right is that he could be eliminated in the first round of the two-stage contest on April 23.

Opinion polls currently show that far-right leader Marine Le Pen and 39-year-old centrist Emmanuel Macron would progress to contest the runoff on May 7.

Fillon was the frontrunner until mid-January when the Canard Enchaine newspaper alleged he paid his British-born wife Penelope and their children nearly €900,000 (S$1.3 million) as parliamentary assistants or advisers.

French lawmakers are allowed to employ family members, but investigators are searching for evidence of what work Fillon's wife did.

He revealed Wednesday he is to meet investigating magistrates to be charged on March 15, sparking a series of defections from his team.

His foreign affairs point man Bruno Le Maire and his campaign spokesman have deserted him and the leader of a small centrist party, the UDI, said it was withdrawing its backing.

The left-leaning Liberation newspaper wrote Saturday: "It is no longer the rats leaving a sinking ship, but the ship leaving the rat."

With just seven weeks to go before voting begins, the entourage of 71-year-old former premier Alain Juppe has said he is prepared to take over, providing Fillon agrees to stand down.

But high-profile Fillon supporters from the business world rallied round him.

Henri de Castries, a former chief executive of the Axa insurance giant, said Fillon should remain in the race because his clear victory over Juppe in November's nominating contest gave him "legitimacy".

"I have never had any doubt about the strength of his character and the depth of his determination," he told Le Parisien newspaper.

Despite a forecast of heavy rain, Fillon's campaign is hoping for a massive turnout on Sunday, helped by the organising prowess of groups such as Manif Pour Tous, which staged a huge protest against gay marriage in October.

Fillon is a devout Catholic and his surprise win in the Republicans' primary was widely attributed to his conservative social views.

Juppe, his possible replacement, is a former premier who was given a suspended jail sentence in 2004 over a party funding scandal.

He is more centrist than Fillon and believes France needs to regain its "happiness".

Juppe has said he would only replace Fillon if the candidate makes the decision to pull out himself, "and the rightwing and centre camps... have to be united behind him", a source in Juppe's camp told AFP.

One poll on Friday showed Juppe would vault into the lead if he stood.

Juppe would have 26.5 per cent of votes, giving him a narrow lead over Macron on 25 per cent, while Le Pen would slip to third place on 24 per cent, according to the Odoxa-Dentsu Consulting survey of 943 people.

Le Pen, 48, campaigning on an anti-immigration and anti-EU platform, has sought to capitalise on the same anti-establishment sentiment that propelled US President Donald Trump to power and led to the British vote to leave the European Union.

Polls currently show however that the leader of the National Front (FN) will be beaten in the second round by either the fast-rising Macron or the conservative candidate.

Other candidates, such as leftist Socialist Benoit Hamon, are angry that Fillon's scandal is overshadowing the campaign.

"We can't debate the issues," he complained.