All 11 candidates ranging from centrist Emmanuel Macron, who is narrowly leading the field, to minnows like Philippe Poutou, a Ford factory mechanic polling at 1.5 percent, will be interviewed on France 2 television for 15 minutes each.
With just three days to go until the first round on Sunday, the race -- which could decide the future of the European Union -- has tightened dramatically.
Opinion polls show Fillon and Macron, both pro-EU, locked in a tight four-way contest with far-right leader Marine Le Pen and hard-left firebrand Jean-Luc Melenchon, both eurosceptics, making it the most unpredictable election in years.
Three surveys showed Macron having a slight edge over Le Pen with 23-25 percent against 22-23 percent.
Fillon and Melenchon were tied at around 19 percent, after a late spurt that has put them within striking distance of the frontrunners.
The top two will advance to a run-off vote on May 7.
The spectre of an EU-bashing final between Le Pen and Melenchon -- one of six possible line-ups -- has caused nervousness among investors and thrust Europe to the top of the agenda.
Le Pen is proposing to a referendum on France's membership of the EU after negotiations with Brussels on returning most key powers to national capitals. She also wants to scrap the euro and bring back the French franc.
Former prime minister Fillon has tried to rebound from an expenses scandal by presenting himself as a safe, experienced pair of hands at a time of deep global uncertainty following Britain's decision to leave the EU and Donald Trump's rise to the White House.
The 63-year-old candidate for the conservative Republicans party was the early leader in a race that had been seen as a walkover for the right after five years of troubled Socialist rule.
But he lost ground after being charged over accusations that he put his wife Penelope on the public payroll for a fictitious job as his parliamentary assistant, for which she was paid nearly 700,000 euros ($750,000).
'Poison of terrorism'
Fillon used the arrest of two men apprehended in Marseille on suspicion of planning an attack on the election as an illustration of the dangers France faces -- and the potential weaknesses of his opponents.
He aimed particular criticism at 39-year-old Macron, who spent two years as economy minister in the current Socialist government but has never held elected office.
"On the fight against radical Islam, as with everything else, Macron is vague," Fillon told the right-leaning Le Figaro newspaper on Thursday.
Fillon confirmed that he had been warned by police last week that he had been earmarked as a "target" by jihadists.
Although prosecutors have refused to say if the two men arrested were aiming to attack a particular candidate, Fillon said of himself that it was "not out of the question that the candidate who has the most radical plan to take on Islamic terrorism be the target".
He also returned to a message he has hammered home in recent weeks -- that he alone among the leading candidates could secure a "strong" majority in the legislative elections that follow in June.
Le Pen has also sought to capitalise on the arrests in Marseille, accusing her rivals of turning a blind eye to Islamic terrorism.
Addressing 5,000 flag-waving supporters in Marseille, the city where the men were detained, on Wednesday, she said: "I have been denouncing this terrible poison of Islamic terrorism since I launched my campaign... and none of my rivals are willing to debate the subject.
"They wanted to stay quiet about this problem, to suppress it, to keep it at a distance like one sweeps dust under the carpet."
Le Pen, 48, has spent years trying to build support for the National Front (FN) by campaigning on bread-and-butter issues such as the economy, but in the final days of the race she has returned to the party's stock themes of national security and immigration.
On Wednesday, she repeated that she would slash immigration, make it harder to obtain French nationality and crack down on suspected Islamists.