The former prime minister's attacks have drawn criticism from President Francois Hollande who said late on Thursday that the rally near a central Paris square dedicated to human rights should not go ahead.
"You will see there will be tens of thousands," said conservative senator Bruno Retailleau on Europe 1 radio. "Sunday will be an important date, an important moment."
Allegations that Fillon paid his wife hundreds of thousands of euros in taxpayers' money for work she may not have done have damaged his reputation as an honest politician and blown the one-time election favorite's campaign badly off course.
Opinion polls show far-right leader Marine Le Pen and independent centrist challenger Emmanuel Macron as the frontrunners, with the former investment banker and ex-economy minister favorite to win a runoff vote.
Fillon this week repeated his denials of wrongdoing and has stepped up his attacks against the judiciary.
"I have been singled out by the judicial system. It's as if I had to be brought down at all costs," he said in an interview with the newspaper Midi Libre on Thursday.
His attacks and sliding popularity have caused some of his key supporters to abandon his campaign.
Hollande, on a visit to Corsica on Thursday, called for responsibility from the Fillon camp and the demonstration to be called off.
"We can't have demonstrations in our country, in our Republic, that question our institutions, the work of the judiciary or of the police in the course of an inquiry," the president said.
The demonstration was organized after Fillon revealed he would be put under formal investigation, but organizers have since said it is in support of his candidacy and is not an attack on the judiciary.
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