French left-winger Benoit Hamon has won the Socialist nomination for president in this year's election, according to partial results, easily beating centrist ex-prime minister Manuel Valls in a run-off.

Results on Sunday from 60 percent of polling stations showed Hamon winning 58.65 percent to 41.35 percent for Valls, Thomas Clay, the head of the primary organising committee, said.

In a concession speech, Valls congratulated his opponent and wished him good luck in the presidential vote that will take place in April and May.

"Benoit Hamon won decisively," Valls said. "Benoit Hamon is henceforth the candidate of our political family."

Turnout in the vote was "much higher" on Sunday than during the first round a week ago, when 1.6 million people cast ballots, Clay said.

'Universal income'

Hamon's victory is another upset in an election seen as highly unpredictable, with the 49-year-old former education minister viewed as an outsider only three weeks ago.

Hamon has pitched himself as a man of fresh ideas for the left, promising to bring in universal basic income - a state handout to all adults, irrespective of income - and new environmental protections.

The cost, according to Hamon, will be around 350bn euro ($374.82bn), roughly equivalent to the annual budget of Europe's second-biggest economy. An ambitious overhaul of taxes will be pursued to fund it, he says.

He also wants to levy taxes on robots, legalise cannabis, introduce stricter rules on chemical products and introduce a new corps of state inspectors to combat discrimination.

He won a first round of the primary last Sunday and picked up important endorsements from rivals this week.

Valls, prime minister under Hollande until December, lampooned his programme as unworkable and labelled him him a dreamer.

Fillon's troubles

Hamon's nomination completes the line-up of the main candidates in the upcoming two-round election, which pollsters forecast will confirm France's shift to the right after five years of unpopular Socialist rule.

The candidates include right-wing Republicans party frontrunner Francois Fillon, far-right candidate Marine Le Pen, as well as centrist Emmanuel Macron, who some analysts see as likely to benefit from the Socialists' tilt left.

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Fillon, who was also a long shot until he clinched the Republicans nomination in November, has consistently been tipped to become France's next leader.

But his campaign has been in turmoil since last Wednesday when a newspaper reported his wife had been paid around 500,000 euro ($540,000) over eight years for a suspected fake job as a parliamentary aide.

Those allegations have sparked a preliminary judicial inquiry, but there was more bad news for Fillon on Sunday.

Investigative website, Mediapart, and the Journal du Dimanche newspaper reported Fillon had used his parliamentary allowance to pocket up to 25,000 euro ($26,800) while working as a senator.

In a defiant speech on Sunday in front of thousands of supporters in Paris, Fillon said he would not let himself be "intimidated".

"It's more than me as a person that is in the crosshairs, it's a higher idea of France that they want to take down mid-flight," he said.