Polling stations opened Sunday morning for the first round of a primary organised by France's ruling Socialist Party to pick a centre-left candidate for this year's presidential election. The Socialists were pioneers in holding primaries in France in 2011 but this time they expect a lower turnout than in the mainstream right's primary last month.

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Seven candidates are looking to clinch the nomination to be the candidate in France’s presidential elections in April and May this year.

Four of them are from the Socialist Party and three are members of other parties that back the government.

The candidates include a philosopher, a former prime minister, an ecologist and just one woman.

To read our profiles of the seven candidates click here

But only three are in with a real chance.

The clear frontrunners are former prime minister Manuel Valls, who is pro-business and on the right of the party, and two left-wing rebels - Arnaud Montebourg and Benoît Hamon, whose proposal of a universal basic income has been the main point of controversy in the primary TV debates.

Montebourg and Hamon are both former ministers who left the government because they found President François Hollande’s economic policies too right-wing.

Fewer polling stations than 2011

Voters will each pay one euro to cast their vote but there's total suspense over how many people will turn out, given the disappointment on the left in the Socialists' record in government.

The French primaries organising committee predicts 1.5 million voters at most, half the number that voted in the Socialist primaries in 2011, which Hollande won.

As a result there are 2,000 fewer polling stations than in 2011.

There is one encouraging sign, however: 16,000 French ex-pats have registered, three times more than for the last Socialist primaries.

If no candidate gets more than 50 percent of the vote in the first round, a runoff will be held on 29 January between the two candidates with the most votes.

Macron, Mélenchon push Socialists to fifth place

But the real debate on the French left is taking place outside the primaries and outside the Socialist Party itself.

It is between former economy minister Emmanuel Macron, a pro-business former investment banker, and the firebrand Jean-Luc Mélenchon, another former minister, who left the Socialist Party in 2008 and has the Communist Party's backing, on the hard left.

They have managed to relegate the Socialists to an embarrassing and demoralising fifth place, but are themselves at present behind the National Front's Marine Le Pen and socially conservative François Fillon.

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