By Ingrid Melander and John Irish

PARIS (Reuters) - An assertive Francois Fillon told French voters on Thursday that rival Alain Juppe "does not really want to change things," in a debate a flash poll showed Fillon winning, days before a vote to decide which one will run in the 2017 presidential election.

The winner of Sunday's conservative primaries' vote will have a good chance of being elected president in May, considering the divisions on the left and opinion polls showing a majority of voters opposed to seeing the far-right in power.

"Alain Juppe does not really want to change things. He's staying within the system, he just wants to improve it," Fillon said in the televised debate on Thursday. "My project is more radical."

Fillon, 62, a social conservative who champions free-market economic policies, unexpectedly stormed to first place in the seven-strong primaries' first round last Sunday, with a strong lead on 71-year-old Juppe, who had been the favorite in opinion polls for months. Both are former prime ministers.

In an online survey by Elabe pollsters of 908 people who watched Thursday's debate, 71 percent of conservative and center-right voters found Fillon, an admirer of late British Prime Minister Margaret Thatcher, hardly a popular figure in France, more convincing than Juppe.

Fillon was also ahead among all viewers independent of their political stripe but by a smaller margin, with 57 percent versus 41 percent for Juppe.

Both propose supply-side economic strategy with cuts in public spending and raising the retirement age.

They differ on other major issues, both on the economic and social fronts, with Fillon proposing more public-sector job cuts, which Juppe deems unrealistic, and a longer working week.

"Reform should not be a punishment but bring hope," said Juppe, who is backed by France's two main centrist parties and is relatively popular among left-wing voters after campaigning on an inclusive, "happy identity" platform. During the debate, Juppe insisted on his attachment to France's diversity.

"The French social model exists, I want to consolidate it," Juppe said, referring to the country's welfare safety net. "We should not break it."

WHO WILL VOTE?

Anyone can take part in the primaries, whether or not they are members of the Les Republicains conservative party.

One of the unknowns is whether left-wing voters, whom pollsters said made up at least 10 percent of first-round voters as they sought to block former President Nicolas Sarkozy by casting ballots for Juppe, will show up again this Sunday, and if so, how many.

Fillon trailed Juppe and Sarkozy for months in opinion polls, and the possibility of his victory on Sunday changes the equation in the center and left of the French political landscape.

Officials, including government spokesman Stephane Le Foll, have said they have no doubt that Socialist President Francois Hollande will seek a new mandate despite his record-low popularity.

He is expected to make his decision known by Dec. 7, and a source close to Hollande told Reuters a Fillon victory would be good news for him.

"The problem with Juppe - both his personality and his proposals - is that left-wing voters don't fear him," the source said on condition of anonymity. "With Francois Fillon, everybody will say: 'Oh la la, he's really right-wing'."

A survey by Elabe pollsters on Wednesday showed that while conservative and center-right voters overwhelmingly preferred Fillon's policies to Juppe's, the latter candidate was more popular among the French overall - a sign that Fillon could struggle more than his rival in the presidential election.

Polls for months have predicted far-right National Front party leader Marine Le Pen would qualify for the second round of the presidential election but lose the run-off. There have been no recent polls testing her against Fillon, but a poll in September showed him defeating her.

(Writing by Ingrid Melander; Editing by Peter Cooney)