"No one today can prevent me being a candidate," Fillon said in a TV interview in which he again said a probe into allegations he gave his wife a highly-paid fake parliamentary job was politically motivated.
Earlier on Sunday, the under-pressure French conservative Francois Fillon apologised to his supporters for the fake jobs scandal that threatens to sink his presidential bid but said he was sure he would be proved innocent.
In a speech closely watched for signs he could abandon his campaign, Fillon told a crowd of tens of thousands near the Eiffel Tower that he had made a "mistake" by employing his wife Penelope as his parliamentary assistant.
Fillon, who is to be charged this month over claims he paid British-born Penelope and their children nearly 900,000 euros ($950,000) of public funds for doing little work, said he was convinced he would eventually be found not guilty.
WATCH: Protests erupt in Paris over election campaign
"The problem is that by then it will be too late, the election will have been skewed," 63-year-old Fillon said, as British-born Penelope watched him from the stage.
Some senior lawmakers in his Republicans party have called for Fillon to be replaced immediately, with polls currently suggesting he would be eliminated in the first round of the election on April 23.
Polls show far-right leader Marine Le Pen and 39-year-old centrist Emmanuel Macron would progress to the runoff on May 7.
Former prime minister Alain Juppe, 71, has indicated to AFP through his entourage that he could be ready to replace Fillon as the rightwing candidate.
Supporters of conservative presidential candidate, Francois Fillon, listen during a rally in Paris on Sunday, March 5, 2017. Source: AAP
'Examine your conscience'
The Republicans party leadership is to meet on Monday evening.
Addressing his words to them at the rally, he said: "I have examined my conscience, now it is time for you to examine yours."
Fillon is to appear live on French television news at 1900 GMT Sunday. His team had earlier cancelled the interview before saying it would go ahead after all.
The danger for the conservatives is that an election they once expected to win easily could slip away if Fillon remains in place.
Fillon had been leading the race until Le Canard Enchaine newspaper made the fake jobs allegations in mid-January. His support has slipped considerably since then.
Fillon told the crowd, who were drenched by rain at one point, that he had been "attacked by everyone" and tweeted on Sunday "they want me to be alone."
But he admitted he bore some responsibility if his campaign "was confronted by such formidable obstacles".
He apologised to his supporters, saying that among the judicial investigation "you have been forgotten".
Penelope broke her silence over the scandal earlier Sunday, telling Le Journal du Dimanche she had carried out "a lot of different tasks" for her husband, a former prime minister, during his career.
She had also urged him to "keep going to the end" but said only he could make the final decision.
French lawmakers are allowed to employ family members, but investigators are looking into what work Penelope did after it was claimed she did not have a pass for the National Assembly building.
"He needed someone to do a lot of different tasks, and if it wasn't for me, he would have paid someone to do it, so we decided it would be me," Penelope told the paper.
Francois Fillon has claimed the accusations are politically motivated, even suggesting he believes the ruling Socialist government is behind the investigation.
Pressure grew on Fillon after police had raided his country manor house near Le Mans on Friday. His Paris apartment was searched on Thursday.
'Mr Clean'
Fillon was a surprise winner of the conservative nominating contest in November, campaigning as a "Mr Clean" unsullied by his opponents' legal difficulties and pledging to slash 500,000 civil servants' jobs.
But the claims about his expenses have led to barbs that his moral authority has been undermined.
Fillon is to meet investigating magistrates on March 15 and be charged. His wife is also to face charges.
His foreign affairs point man and spokesman have quit his campaign and the leader of the small centrist UDI party said it was withdrawing its backing.
Juppe, who is more centrist than Fillon, was given a suspended jail sentence in 2004 over a party funding scandal.
"Examine your conscience", French conservative presidential candidate, Francois Fillon, says while delivering his speech during a rally in Paris. Source: AAP
He lost the primary to Fillon, but a poll Friday showed he would vault into the lead if he stood.
The financial accusations have added to an already unpredictable race.
Le Pen, 48, campaigning on an anti-immigration and anti-EU platform, has sought to capitalise on the anti-establishment sentiment that propelled US President Donald Trump to power and led to Britain's vote to leave the European Union.
Polls currently show however that National Front leader Le Pen will be beaten in the second round by either the fast-rising Macron or the conservative candidate.
