Fillon denounced an "institutional coup d'état" organised by "the left", at a hastily organised meeting to rally MPs from his Republicans party on Wednesday morning.
"We know where this affair comes from, it comes from the government, it comes from the left," he said, according to participants.
Government spokesperson Stéphane Le Foll dubbed the accusation "unacceptable" and called on Fillon to let the legal system do its work.
Disarray in mainstream right ranks
Although Fillon's closest allies denounced a "media lynching", many other right-wingers were reconciled to the idea that another candidate has to be found for the April-May presidential election.
The results of the primary, in which Fillon won a surprise victory on a Mr Clean ticket, are now redundant, Georges Fenech, a supporter of former president Nicolas Sarkozy, said on Wednesday and several other MPs admitted there was panic in the ranks.
Fillon begged them to stay judgement for a fortnight, saying that the investigation's results should be made public by then.
An opinion poll taken after the latest revelations showed him pushed into third place, leaving centrist former economy minister Emmanuel Macron to face the far right's Marine Le Pen in the second, deciding round.
Pay rise for Penelope
The alternate who replaced Fillon as MP when he became labour minister in 2002 was interviewed by investigators on Wednesday.
Marc Joulard, who is now mayor of Sablé-sur-Sarthe and an MEP, has to explain why he continued to employ Fillon's wife, Penelope, who is alleged to have done no work despite being employed as a parliamentary assistant, and even gave her a pay rise in 2007 that meant she took home more than he did.
Also on Wednesday, Le Monde newspaper and the Mediapart website called on the former prime minister to explain how a consultancy he runs came to earn 757,526 euros between 2012 and 2015, virtually all of the money going into his pocket.
Fillon set up 2F Conseil on leaving office in 2012, just 13 days before returning to parliament, thus dodging a ban on MPs setting up consultancies during their mandate.
Its turnover since then has been 1,015 million euros, Le Monde reports, and most of its profits are paid into Fillon's bank account.
But, apart from 40-60,000 euros from a company run by René Ricol, who worked for the government while Fillon was prime minister, the sources of the money are unclear.
The media want to know whether any foreign governments paid him as a consultant, pointing to visits he made to Lebanon, Kazakhstan, whose relations with certain French politicians have come under scrutiny in the Kazakhagate affair, and Russia.
Fillon has called for the European Union to end sanctions against Russia and praised its role in the Syrian conflict.
"François Fillon works for no foreign government and no foreign company," declared his PR respresentative Anne Méaux.
On Wednesday Fillon called off visits to Lebanon and Iraq, planned for this weekend, saying he wanted to "stay in contact with his troops and the French people".