After two hours of debate on Tuesday night among the 11 candidates on unemployment, immigration and relations with Europe, two far-left candidates took the gloves off and challenged Fillon and Le Pen on judicial investigations against them.
"Since January, it's just been a great campaign ... the more we dig, the more corruption there is, the more cheating there is," Philippe Poutou, a fringe candidate, said, Al Jazeera reported.
He was referring to a press report in the satirical Le Canard Enchaine weekly which was the first to allege that Fillon had been paying his wife huge sums of taxpayers' money for work she had not properly carried out.
Fillon, a 63-year-old former conservative prime minister, and his wife are being investigated over the allegations, though they deny any wrongdoing.
Looking ill at ease, Fillon replied that he had made no mistakes and would not take questions on the allegations.
"I won't be intimidated," he said.
"I'll take you to court for that," Fillon said.
Fillon was once frontrunner in the election, but after the fake jobs allegations emerged his ratings fell and he is now trailing in third place in the election first round, according to opinion polls, which would mean elimination.
Poutou, a car factory worker who is expected to get only minimal support in the April 23 first round of the election, also had harsh words for Le Pen.
The National Front leader in February used her immunity as EU legislator to refuse to go to a police summons over allegations that she had made illegal EU payments to her staff.
"When you're summoned by police, you go. There is no workers' immunity," said Poutou in a comment widely picked up on Twitter.
He was supported by another far-left candidate Nathalie Arthaud, who said: "Supermarket cashiers can be fired just for stealing a voucher. There is a shocking discrepancy here."
Le Pen, who like Fillon has also denied wrongdoing, angrily responded, saying she was the victim of "political persecution".
"Is this a [police] interrogation? I thought this was a debate but it looks like the prosecutors are here," she said.
Benoit Hamon, the Socialist Party candidate, who is unlikely to get beyond the election's first round, mocked Le Pen for "playing the victim".
Earlier on Tuesday, a judicial source said a separate investigation into the National Front's finances in France's northernmost region was under way.
Le Canard Enchaine said the inquiry targeted Le Pen's campaign director, David Rachline.