Far-rightist Marine Le Pen threw France's presidential race wide open on Sunday by scoring nearly 20 percent in the first round - votes that may determine a runoff between Socialist favorite Francois Hollande and conservative President Nicolas Sarkozy.

Hollande led Sarkozy by about 28.5 percent to 26 percent in reliable computer projections broadcast with one-third of ballots counted, meaning the two will meet in a head-to-head decider on May 6 that may be closer than had been expected.

But Le Pen's record score of 19.6 percent was the sensation of the night, beating her father's 2002 result and outpolling hard leftist Jean-Luc Melenchon in fourth place on 11 percent. Centrist Francois Bayrou finished fifth on less than 9 percent.

It was the first time a sitting president seeking re-election had been beaten into second place in the first round. But Sarkozy backers at his campaign headquarters chanted "We are going to win", interpreting Le Pen's score as more significant than Hollande's narrow lead over the incumbent.

Before voting, opinion polls had suggested a comfortable win for the Socialist in the second round.

Le Pen, who took over the anti-immigration National Front in 2011, wants jobs reserved for French nationals at a time when jobless claims are at a 12-year high. She also wants France to abandon the euro currency and restore monetary policy to Paris.

"This first round is the start of a vast gathering of right-wing patriots," she told cheering supporters at her campaign headquarters, without endorsing either of the finalists.

"Nothing will ever be the same again."

Le Pen's unexpectedly high score reflected a surge in anti-establishment populist parties in many eurozone countries from the Netherlands to Greece as austerity and the debt crisis bite.

Jean-Marie Le Pen, visibly elated at his daughter's result, said the National Front would now focus on winning seats in June parliamentary elections. "There is a lot of hope for us," he told France 2 television as party supporters shouted "Victory!"

Voter surveys show about half of Le Pen's supporters would back Sarkozy in a second round and perhaps one fifth would vote for Hollande, making her a potential kingmaker in the runoff.

NOTHING 'IN THE BAG'

"Nothing is in the bag yet," said Foreign Minister Alain Juppe, one of Sarkozy's closest aides.

The elder Le Pen's 16.9 percent score in the 2002 first round caused a political earthquake, knocking then Socialist Prime Minister Lionel Jospin out of the runoff and forcing left-wing voters to rally behind conservative Jacques Chirac.

Sarkozy, 57, has painted himself as the safest pair of hands to lead France and the euro zone in turbulent times, but Sunday's vote appeared to be a strong rejection of his flashy style as well as his economic record.

If Hollande wins on May 6, joining a small minority of left-wing governments in Europe, he has promised to lead a push for a bigger focus on growth in the eurozone, mainly by adding pro-growth clauses to a European budget discipline treaty.

The prospect of a renegotiation of the pact is causing some concern in financial markets, as is Hollande's focus on tax rises over austerity at a time when sluggish growth is threatening France's ability to meet deficit-cutting goals.

STYLE HATED

France's sickly growth, along with its stubbornly high unemployment, are major factors hampering Sarkozy's battle to win a second term, despite an energetic campaign against the blander but more popular Hollande.

Melenchon, whose clench-fisted call for an anti-capitalist revolution made him the most colourful figure on the campaign trail, called on left-wing voters to fight back and make sure Sarkozy is ousted next month.

"I call on you to come out on May 6 and beat Sarkozy without asking for anything in exchange. I urge you: don't drag your heels, mobilise as though it were me you were sending to victory in the presidential election," he said.

If Sarkozy loses, he would be the 11th euro zone leader to be swept out since the start of the bloc's debt crisis in late 2009 and the first French president to lose a re-election bid in more than 30 years. A deep dislike of a personal style seen as arrogant and vulgar drove many to vote against him.

"France needs a radical change of direction, mainly on the economy," said Jean-Noel Harvet, a public sector worker voting earlier on Sunday in the northern town of Cambrai.

Hollande, 57, promises less drastic spending cuts than Sarkozy proposed and wants higher taxes on the wealthy to fund state-aided job creation, in particular a 75 percent upper tax rate on income above 1 million euros ($1.32 million).

If he wins, he would be only France's second left-wing leader since the founding of the Fifth Republic in 1958, and its first since Francois Mitterrand, who beat incumbent Valery Giscard-d'Estaing in 1981 and ruled until 1995.

Hollande had called on his supporters to take nothing for granted, mindful of the fiasco for the left in 2002 when record abstention saw the Socialist Jospin pushed out in the first round by the elder Le Pen.

Turnout ended up at a healthy 78.7 percent.

A swing to a Socialist government in France could alter the direction of Europe at a time when worries are resurfacing over the euro zone debt crisis and in particular over France's strained public finances, which already triggered a downgrade of Paris's triple-A credit rating by Standard & Poors in January.

Sarkozy has played up his credibility as an economic steward after he helped steer the euro zone through the worst of its crisis last year. Hollande has blamed him for the parlous state of France's public finances and for the rating downgrade.

Some investors see a risk that Hollande's focus on tax rises over spending cuts, his slower timetable for balancing the budget and his plan to raise taxation on the financial sector, could drive up French bond yields.

However, many economists see both Hollande and Sarkozy being obliged to pursue fairly similar, fiscally tight, economic policies. Differences between the two on France's wider European and foreign policies are also seen as relatively limited.