PARIS — François Fillon won France’s first-ever conservative presidential primary Sunday after promising drastic free-market reforms and a crackdown on immigration and Islamic extremism, beating a more moderate rival who had warned of encroaching populism.

“President! President!” chanted the former prime minister’s supporters as he declared victory over Alain Juppé in a nationwide runoff election.

Polls suggest the sober, authoritative Fillon, 62, would have a strong chance of winning the French presidency in the April-May election, amid widespread frustration with France’s current Socialist leadership.

Fillon, who was prime minister from 2007-2012 under ex-President Nicolas Sarkozy, enjoyed a surprise surge in popularity in recent weeks. A rise in nationalist sentiment across Europe may have favored his strict conservative positions over Juppe’s more centrist stance.

France needs “a complete change of software,” Fillon said, promising in his victory speech to defend “French values.”

Among his promises: slash public spending, cap immigration, support traditional family values and reach out to Russian President Vladimir Putin.

Juppé, 71, congratulated Fillon on his “wide victory.” During the primary campaign, he expressed similar ideas as his rival on the French economy, but tried to rally conservatives around a more tolerant attitude toward France’s ethnic, religious and social diversity.

With results from 96 percent of voting stations, organizers of the Republicans’ party primary said Fillon had 66.5 percent of votes and Juppe 33.5 percent. More than 4 million of France’s 44 million voters took part, which was considered a good turnout given that it was the conservatives’ first experiment with a primary.

Fillon’s toughest challenge ahead is likely to be far-right leader Marine Le Pen. Le Pen, candidate of her National Front party, is running an anti-establishment campaign that particularly targets immigrants, France’s Muslim minority, and the European Union.

Socialist President François Hollande is expected to announce in the coming weeks whether he will seek re-election, but the French left has been deeply weakened by his extreme unpopularity. A wild card is outsider candidate Emmanuel Macron, Hollande’s former economy minister, who is leading a centrist campaign.

Fillon walked a careful line Sunday, stressing the need for “authority of the state” but also insisting, “No one should feel excluded from a society that I want to see more just and with more solidarity.”

If elected next year, he pledges to hold a referendum on a quota system for immigrants to reduce legal immigration “to a minimum,” and to push for stronger controls at Europe’s borders.

In a country still rattled by a string of deadly Islamic State group attacks, Fillon wants to prohibit French jihadists from returning home. He recently published a book called “Conquering Islamic Totalitarianism.”

A practicing Catholic with a British wife of 36 years and five children, Fillon pledges to weaken adoption rights for same-sex couples. Yet he has said he wouldn’t scrap a 2013 law allowing same-sex marriage.

His most dramatic proposals concern France’s long-stagnant economy, beset by chronic 10 percent unemployment.

He wants to cut taxes on businesses, slash public spending by 110 billion euros ($116 billion) and reduce the number of public servants. He would also raise the retirement age from 62 to 65, extend the workweek beyond 35 hours, and ease France’s strict labor rules in order to boost job hiring.

Soccer agent Cherif Diallo said it was Fillon’s economic program that won his vote.

“In life, you must sacrifice in order to obtain good results. The program of François Fillon is radical as his adversaries say, but it’s a necessity in order to get the country in order,” Diallo said.

While he’s seen as lacking charisma, Fillon’s supporters like him because they regard him as experienced and well-qualified for France’s top job.

Besides serving as prime minister, he’s been a cabinet minister six times and spent years as a lawmaker representing his hometown of Le Mans in western France, home to the famed 24 of Le Mans auto race.

Both Fillon and Juppé, who campaigned on similar economic platforms, are high-profile leaders of the Republicans’ party who knocked Sarkozy — their former boss — out of the primary’s first round of voting a week ago. Sarkozy then threw his weight behind Fillon.

Sunday’s runoff came after a bruising and highly adversarial end phase to the months-long primary contest, an American-style effort to end party infighting and bolster support for the party’s nominee. The conservatives previously chose their candidate internally.

Fillon has said he wants to drop sanctions against Russia over its aggressive actions in Ukraine and partner with Russia in the fight against Islamic State extremists. Fillon insists “Russia poses no threat” to the West.

All French citizens over 18 — whether they are members of the Republicans’ party or not — were eligible to vote in the primary, if they paid 2 euros in fees and signed a pledge stating they “share the republican values of the right and the center.”

Thomas Adamson and Jeffrey Schaeffer in Paris contributed to this report.