Fillon is up against another former prime minister, Alain Juppe, who has a week to turn around his momentum-sapped campaign and win over the supporters of the other candidates, Reuters reports.
At stake is an almost certain place in the second round of next spring's presidential election, pollsters say, with the French left in turmoil under the deeply unpopular President Francois Hollande. There the conservative challenger would in all likelihood face the leader of the resurgent far-right National Front party, Marine Le Pen.
Read alsoEurope at risk of collapse; France, Germany must lead - French PM"I believe more than ever that the people of France need to come together to turn the page of a disastrous five-year term that has demeaned our country and to block from power the National Front which would lead us into the worst of adventures," a downcast-looking Juppe told his supporters.
A snap poll by Opinionway showed Fillon winning next Sunday's head-to-head contest with 56% of support.
