The legal woes of Fillon, a conservative who had led the race for weeks, have benefited centrist former economy minister Emmanuel Macron in a increasingly close race for the Elysee Palace.
Fillon's problems are the latest twist in an election being watched closely across Europe after the victory of the Brexit camp in Britain and Donald Trump's win in the United States.
A poll Sunday indicated Marine Le Pen, the leader of the anti-immigration and anti-EU National Front (FN), would score highest in the first round on April 23, with Fillon and Macron almost neck-and-neck behind her.
Le Pen is currently forecast to be beaten in the runoff on May 7, but after a year of political shocks the polls are being viewed with caution.
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No proof
Fillon and his Welsh-born wife Penelope were quizzed separately by investigators for several hours on Monday over claims in the Canard Enchaine newspaper that she received payments for several years as his parliamentary aide.
While lawmakers are entitled to employ family members, the paper said it could find no proof she did any work in return for the money.
The couple were also questioned over money received for Penelope's work at a literary review owned by a billionaire friend of Francois Fillon.
After Monday's questioning, Fillon's campaign team issued a short statement saying the couple had "provided details needed to show the truth, in order to demonstrate the work done by Mrs Fillon".
Fillon has dismissed the allegations as part of a dirty tricks campaign and told a packed rally in Paris on Sunday that he would not be "intimidated".
"They're trying to take me down, through Penelope," he said.
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But the scandal has damaged the 62-year-old former prime minister, who emerged as the surprise winner of the rightwing nomination after campaigning as a "clean" candidate and proposing to slash public spending by cutting the jobs of 500,000 civil servants.
The deeply divided ruling Socialist party chose the hard-left Benoit Hamon as their candidate on Sunday.
Hamon, 49, was a rank outsider just three weeks ago but leftwing supporters weary after President Francois Hollande's troubled five years in power warmed to his radical programme that includes paying everyone a universal basic income of up to 750 euros ($800) a month.
With polls showing Hamon will be eliminated in the first round, he urged the fractured left to pull together to build a "governmental majority".
With some Socialist voters scared off by Hamon's overtly leftist message and Fillon embroiled in the claims against him, former investment banker Macron has picked up support.
Race tightens
A poll published Sunday showed the race tightening.
Le Pen would lead all candidates in the first round with 25 percent, ahead of Fillon on 21 percent, with Macron just one point behind, according to the survey conducted by Kantar Sofres-Onepoint on Thursday and Friday.
Hamon would receive just 13 percent, according to the survey of 1,032 voters.
The poll also showed that while Fillon would easily beat Le Pen in a run-off, he would lose a duel with Macron.
On Sunday, Fillon described 39-year-old Macron, who served as economy minister in Hollande's cabinet, as "the prototype of the elite who know nothing of the reality of our country".
Fillon's team have also highlighted a new book in which Macron is accused of using 80 percent of the annual entertainment budget of his ministry for his own receptions.
Fillon has vehemently denied any wrongdoing by himself or his wife, insisting that when he was an MP in Paris his wife did a lot of constituency work but was based at their chateau near Le Mans in northern France.
Penelope, a mother of five, has always styled herself as a low-key political wife who preferred to remain in the country rather than in Paris. She has not spoken publicly about the accusations.
