If Fillon beats Alain Juppe in Sunday's primary, he is widely tipped to become the next president with polls showing he would likely face and defeat far-right leader Marine Le Pen in the run-off next May.
Fillon however insisted his vision of France was not as a multicultural country.
When asked whether he saw the future of French society as multicultural he said: “the answer is no”.
“France has a history, a language, a culture, of course this culture and language have been enriched by the contributions of foreign populations, but it remains the foundation of our identity,” he said.
When asked during Thursday night's debate if France was already a multicultural country Fillon said “No, in any case it's not the choice we made, we did not make the choice of communitarianism and multiculturalism.”
“When we go to somebody’s house, we don’t try to take power,” said Fillon adding that immigrants must respect France's cultural heritage.
His rival Juppé disagreed saying “the identity of France was diversity.”
Fillon, the surprise runaway winner in the first round of the rightwing contest last weekend, underlined his intention to reform a country he said was “on the verge of revolt”.
“It is true that my project is more radical and perhaps more difficult,” said Fillon, whose economic ideas have been compared to those of late British prime minister Margaret Thatcher.
Fillon, a 62-year-old former prime minister, wants to slash an eye-popping 500,000 public sector jobs over five years and scrap the 35-hour working week in a bid to kick-start the sluggish French economy.
Fillon said his opponent, the 71-year-old centrist Juppe, would not go far enough. “He doesn't really want to change things, he wants to improve certain things but I'm sure it won't be enough.”
Juppe responded that cutting that many civil servants' jobs “would not be possible”. He favoured “deep and credible” reforms without what he called the “brutality” of Fillon's proposals.
'Putin choosing candidate'
Juppe hit home in the televised debate with a jibe at Fillon's perceived closeness to Russian President Vladimir Putin.
Putin knew Fillon when they were both prime ministers and the Russian president praised him Wednesday as a “great professional” and a “very principled person”.
“This must be the first presidential election in which the Russian president chooses his candidate,” Juppe said.
Fillon brushed off Putin's comments but said the West must work more closely with Russia at a time when relations are at their worst since the Cold War.
“Russia is a dangerous country if we treat it as we have treated it for the last five years,” Fillon said.
He said the real danger to Europe was not Russia, it was the economic threat of “the Asian continent”.
Television viewers gave victory in the debate to Fillon, reflecting a poll Wednesday that showed he would win 65 percent of votes on Sunday against 35 percent for Juppe.
Former president Nicolas Sarkozy was eliminated in a shock result in the first round.
Juppe and Fillon, a devout Catholic, have clashed this week over the latter's attitude to abortion.
But Fillon protested Thursday he had been wrongly portrayed as a “conservative from the Middle Ages” and said he had no intention to reform abortion law even if he personally was opposed to it.
“My conscience is my business,” he said.
Le Pen is currently forecast to come first or second in the first round of the election on April 23 with around 30 percent of the vote, but then fail in the run-off on May 7.
But following the wave of populism that led British voters to choose to leave the European Union and swept Donald Trump to victory in the United States, no-one is writing off the National Front (FN) leader's chances.
Fillon dismissed suggestions his conservative approach made it hard for voters to distinguish between him and Le Pen.
“I have always fought the National Front,” he said, adding: “We have to prevent Madame Le Pen from reaching the second round.” If she did, it would be the sign of an “ailing democracy”.
Le Pen says she wants to ditch the euro and organise a referendum on France's EU membership — a move that would put the future of European integration at stake.
A re-election bid by deeply unpopular Socialist President Francois Hollande seemingly moved closer on Thursday after figures showed a slight fall in the number of unemployed in October.
Hollande has said he would only stand again if he could make a “credible” reduction in unemployment by the end of his mandate. He said the figures were proof his approach was “bearing fruit”.