Police raided the Paris home of French rightwing presidential candidate Francois Fillon on Thursday over an alleged fake job scandal as a senior party colleague warned him he risked dragging his party "into an abyss".

Fillon revealed Wednesday he is set to be charged over allegations he paid his wife and children hundreds of thousands of euros for fake parliamentary jobs, but he has vowed to continue his bid for power.

After searches at his parliamentary office last month, police raided his central Paris home Thursday as he visited winegrowers on the campaign trial in southern France.

The raid "finished several hours ago", a source in Fillon's team told AFP late Thursday, confirming information first reported by Le Parisien newspaper.

Fillon was accused by Dominique de Villepin, another former premier from his Republicans party, of driving the right wing "into the abyss" with his insistence on running for the presidency.

"Going down this dead-end street is taking the state, our faith in democracy and its fellow travellers hostage," de Villepin wrote in Le Figaro newspaper.

Fillon has called the charges against him "entirely calculated to stop me being a candidate for the presidential election" and has ruled out stepping aside.

But defections from his team and calls from senior Republicans for ex-premier Alain Juppe, 71, to replace him have underlined divisions in his camp.

"The French people back me," Fillon insisted Thursday. "The base is holding."

New polls suggest Fillon is in third place and would win 19-20 percent in the first round of the election on April 23, behind centrist Emmanuel Macron and far-right leader Marine Le Pen.

Thousands still turned out to Fillon's rally in the town of Nimes on Thursday evening and his calculation appears to be that he can close the small gap on his rivals in the remaining two months.

The top two from the first round proceed to a run-off on May 7, which Macron is currently shown winning.

But analysts warn against firm forecasts after a rollercoaster campaign so far.

Le Pen loses immunity

Le Pen's legal woes also deepened as the European Parliament lifted her immunity to allow her to be prosecuted for retweeting images of Islamic State atrocities.

The anti-EU, anti-immigration candidate also faces a separate parliamentary expenses investigation and a campaign financing probe in France -- all of which she, like Fillon, denounces as a plot to thwart her.

The investigation in focus on Thursday concerns graphic pictures including that of a beheaded journalist that Le Pen, an MEP, tweeted in 2015.

They were addressed to a French television journalist who had likened her National Front party to the Islamic State group, leading police to open a probe into "the dissemination of violent images".

"The thing about the judicial affairs for Marine Le Pen and the National Front is that they are not about personal enrichment, while Francois Fillon's family is directly implicated," far-right expert Cecile Alduy from Stanford University told a conference this week.

Macron's agenda unveiled

Macron, meanwhile, promised a "strategy to make public life more ethical" as he unveiled his full programme for the first time.

The 39-year-old said he would ban parliamentarians from employing relatives, bar candidates with criminal records from standing for office, and increase the scrutiny of MPs' expenses.

Speaking late Thursday, he also said he would pull out of the election if he was charged "in the same way, in principle, a minister must leave the government if he is charged".

"To be president is to be the guarantor of institutions -- that is to say someone who guarantees the dignity of our public life," he told France 2 television.

Macron founded his independent movement "En Marche" (On the Move) only last April and has been the main beneficiary of Fillon's woes.

He sought Thursday to take on critics who say his pro-European, pro-business platform lacks substance.

Mixing traditionally rightwing measures such as easing strict labour controls and cutting taxes, he also stressed the need for new investment in public schools and measures to help deprived, high-immigrant areas.

"We are not looking to adapt or reform, but to transform," Macron told hundreds of journalists at a launch event to set out his agenda.

Macron believes France is too small to compete on its own in a globalised world and wants deeper integration between countries that use the euro, which would include the creation of a eurozone budget.

"Our responsibility in the years to come is to be able to rebuild the European dream," he said, adding that he would seek to persuade the zone's richest member Germany to invest in other countries.