Speaking in front of a crowd of thousands in Paris, the 63-year-old conservative embarked on a last ditch attempt to save his candidacy, giving what commentators dubbed «the speech of his life».
In calling the rally, Mr Fillon aimed to show his opponents he still has a large body of dedicated support in spite of being mired in a weeks-long series of corruption allegations.
Meanwhile, his Wales-born wife Penelope Fillon defended having worked for her husband in her first interview since those claims emerged. Newspaper allegations, that she and the couple’s children were paid substantial sums for parliamentary aide work they never carried out, have seen Mr Fillon go from clear favourite to third in the polls behind youthful independent Emmanuel Macron and far-right leader Marine Le Pen.
Speaking to the Journal du Dimanche newspaper, Ms Fillon said: “He needed someone that carried out his tasks. If it hadn’t been me, he would have paid someone else to do it, so we decided that it would be me.»
She said the breaking scandal had felt like being “struck by lightning”.
“This is the worst I have had to live through in my life,» she said. «I was so surprised by the violence and hysteria that I have found refuge in my Welsh stubbornness.
“I did not believe that I was doing politics. I was working for my husband. My role was to help him interact with his constituency as an elected representative.»
On the prospect of her husband stepping down from the presidential race, she said: «I told him that he had to keep going until the end. But the decision will be down to him.»
Ms Fillon’s words of support for her husband come as senior conservative politicians said they will propose an alternative candidate to Mr Fillon as soon as Monday morning.
Christian Estrosi, a senior Conservative politician and a close ally of former President Nicolas Sarkozy, told BFM TV: “In the coming hours, we will propose an initiative.
«We do not have the time to debate who has the most talent. I don’t think any of the forty-somethings in our political movement, who have talent, can take on the role to bring us together.»
«The easiest thing obviously … is the person who came second in the primaries and that quite simply is Alain Juppe,» he said.
Mr Estrosi is not the only one to call for Mr Fillon to leave.
The head of France’s centre-right Union of Democrats and Independents (UDI) party, Jean-Christophe Lagarde, also said he wanted Mr Fillon to quit the election race in favour of former prime minister Mr Juppe. He warned that if Mr Fillon did not stand down, it would mean defeat for the Conservatives.
Speaking on Europe 1 radio, he said: «With Fillon it’s a certain failure. This (rally) is an excess because you don’t put the street up against the (state’s) institutions. Even if there are 200,000 people, to win a presidential election you need 20 million people.
«In the Olympics when the gold medal winner is disqualified then it’s the silver medal holder that takes over.”
Even inside the Fillon camp, support seemed to dwindle.
A poll published in the Journal du Dimanche showed more than 70 per cent of French voters want Mr Fillon to withdraw his presidential bid.
The same survey suggests Mr Juppe, who lost to Mr Fillon in the November party primary, was the best placed to step in. He received a personal approval rating of 64 per cent compared to 29 per cent for Mr Fillon.
It comes after Mr Fillon’s campaign manager Patrick Stefanini announced his resignation – the third aide to quit in quick succession.
Mr Fillon, who is renowned for his reactionary views, also pulled out of an early Monday morning radio appearance that aimed to discuss his campaign.
Europe 1 radio presenter Thomas Sotto tweeted: “Francois Fillon has officially cancelled his appearance on Europe 1’s morning show tomorrow.”
The first round of the French elections will be held on 23 April and all presidential candidates must be formally endorsed by at least 500 elected officials before 17 March.