Former prime minister Fillon, an admirer of Margaret Thatcher, pulled off a stunning upset Sunday, coming from behind to knock his former boss Nicolas Sarkozy out of the race and beating the favourite, Alain Juppe, into distant second.
Fillon will go head-to-head with fellow former premier Juppe in a run-off yesterday that is widely expected to decide France's next leader after a prospective duel next year with far-right leader Marine Le Pen.
His rivals today sought to halt his march, with former centre-right prime minister Jean-Pierre Raffarin slamming his plans to cut half a million civil service jobs and force public servants to work longer hours.
"There is no chance of implementing reforms through brute force," Raffarin, an ally of Juppe, told BFM television, calling them "unworkable".
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An advisor to President Francois Hollande described 62-year-old Fillon as a "red rag to left-wing voters". At the other end of the spectrum, the vice-president of Le Pen's National Front (FN), Florian Philippot, said Fillon's economic proposals were "the most extreme" of all candidates.
Polls show both Juppe and Fillon would easily defeat the far-right leader but her rivals have warned that all bets are off in a country where the anti-elite sentiment that propelled Donald Trump to the White House is running high.
Sarkozy's defeat at the hands of his former premier Fillon, a man he once nicknamed "Mr Nobody", brought the curtain down on the ex-president's attempt to revive his 40-year career.
Around 15 per cent of the four million voters who took part in the open-to-all primary were leftist supporters, an Elabe poll showed. Many are thought to have voted against Sarkozy.
Fillon, a car-racing enthusiast, emerged as a compromise choice between Sarkozy and Juppe, a moderate nine years his senior whose reform agenda is seen by many conservatives as too timid.
Results from over 95 per cent of polling stations on Monday gave Fillon 44.1 per cent of the votes cast, ahead of 28.5 per cent for Juppe and 20.6 per cent for Sarkozy. Four other candidates each scored under five per cent.
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