Benoît Hamon, riding to victory from left-wing obscurity on a radical proposal to pay all adults a monthly basic income, will be the Socialist Party candidate in France's presidential election after handily beating ex-prime minister Manuel Valls in a primary runoff vote on Sunday.

Hamon's win sends the divided Socialists, weakened by the chronic unpopularity of outgoing President François Hollande, into a tough presidential battle behind a candidate with limited government experience and hard-left politics that could alienate some centre-left Socialist voters.

With ballots counted at 60 per cent of polling stations, Hamon had almost 59 per cent of the vote to Valls' 41 per cent. Valls immediately conceded defeat in the face of the result that appeared like a clear sanction of both his and Hollande's polices.

With the ruling party having settled on its candidate, the race for the presidential Élysée Palace begins in earnest, although the outcome of the two-round general election vote in April and May looks increasingly uncertain.

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François Fillon, former French prime minister and leading conservative presidential candidate, has been bogged down by a scandal involving wife Penelope. (Pascal Rossignol/Reuters)

At a campaign rally in Paris on Sunday — where a boisterous crowd gave Penelope Fillon a standing ovation and chanted her name, Fillon said, "We have nothing to hide."

"Through Penelope they are trying to break me," he said. "I will never forgive those who chose to throw us to the wolves."

Wounded Socialist party

A priority for Hamon, a 49-year-old former junior minister and, briefly, education minister, will be to rally the Socialists, split ideologically and wounded by Hollande's five-year tenure as president.

The party is also squeezed by rivals on both flanks. Fiery far-left leader Jean-Luc Mélenchon and centrist Emmanuel Macron, Hollande's former economics minister, are both making hay by appealing to disappointed Socialist voters.

Early polling has suggested the Socialist candidate will struggle to advance to the presidential runoff in May, where far-right leader Marine Le Pen could be waiting, campaigning on anti-Europe, anti-immigration and anti-Islam themes.

Supporters of Benoît Hamon, left, cheer after partial results in the second round of the French left's presidential primary election come in. Hamon won the vote and is now tasked with bringing together a wounded and divided Socialist party. (Christian Hartmann/Reuters)

The turnout on Sunday, at around two million voters, was more robust than in the primary's first round of voting a week ago, but still suggested a lack of enthusiasm among the 44-million French electorate. The primary was open to all voters who paid 1 euro ($1.41 Cdn).

Hamon wasn't as tainted as Valls by Hollande's unpopularity because he rebelled and quit the government in 2014.

Valls served as Hollande's prime minister for more than two years until last December, when it became clear the president couldn't win a second term. But having to defend the government's economic policies and labour reforms against Hamon proved an uphill fight.

Universal income

Hamon's signature proposal for a 750 euros ($1,057.21 Cdn) "universal income" that would be gradually granted to all adults also proved a campaign masterstroke. It grabbed headlines and underpinned his surprise success in the primary's two rounds of voting, first against six opponents and then against Valls in the runoff.

Sharply criticized by Valls as unrealistic and ruinous, the no-strings-attached payments would cushion the French in an increasingly automated future, as machines take their jobs, according to Hamon.

He proposes a tax on robots to help finance the measure's huge costs — by Hamon's reckoning, at least 300 billion euros ($423 billion Cdn) if applied to more than 50 million adults.

Hamon also proposes legalizing cannabis and allowing medically assisted deaths.

'Not a serious candidate'

First-time voter Maayane Pralus said Hamon "has a lot of the youth vote with him, which is sick of the old politics."

People call him utopian, but that's the politics we've been waiting for. - Maayane Pralus, Hamon supporter

"People call him utopian, but that's the politics we've been waiting for," the 18-year-old student said.

Valls, 54, emphasized his government experience. He was prime minister when gun and suicide-bomb attacks killed 147 people in Paris in January and November 2015, and still in office in July 2016 when a man drove a truck into crowds in Nice, killing 86 people.

Supporters of Hamon react after their candidate won the Socialist Party presidential nomination in Paris on Sunday. (Francois Mori/Associated Press)

Such are the left's divisions that some Valls supporters may now shift to Macron's independent run for the presidency.

In such a complex political landscape, some voters cast ballots strategically.

Bernard Biassette, 74, a retired bank worker, voted for Hamon only to eliminate Valls, whom he saw as a greater threat to his hoped-for president — Macron.

Hamon "is throwing money out of the window," Biassette said. "He's not a serious candidate."