Nicolas Sarkozy and Socialist rival Francois Hollande stepped up their battle on Tuesday for the six million votes that went to the far-right in the first round of France's presidential election.

"It's up to me to convince the National Front (FN) voters," Hollandetold Liberation newspaper, arguing that many of them were in fact left-wing and their support for the anti-immigrant, anti-EU party was a protest vote.

Hollande and the right-wing Sarkozy -- who beat off eight other candidates in Sunday's first round -- will now square off in a final round on May 6 that opinion polls say the Socialist will win.

But both candidates know their fate may rest in the hands of the 18 percent of voters who plumped for National Front leader Marine Le Pen, who wants to ditch the euro and who rails against the "Islamisation" of France.

The election has laid bare deep French fears over globalisation, the European debt crisis and austerity measures aimed at stemming it, and national identity.

Hollande gave a lengthy interview to Liberation to explain that the massive FN vote was the expression of "social anger" and that he hoped to be able to bring these millions of disgruntled voters back to the mainstream.

He said voters identified Sarkozy with the European Union's free-market ideology and the austerity measures the bloc has imposed across the continent to try and contain its lingering debt crisis.

Hollande's promised alternative is to ease the drive for budget cuts and emphasise measures for growth to stem France's rising unemployment, which now stands at nearly three million out of a population of 65 million.

The Socialist was due to campaign later Tuesday in Hirson in northern France, where he was to meet factory workers in a region where Le Pen got her second highest score in Sunday's first round.

Sarkozy, who was to hold a campaign rally in Longjumeau in the Paris suburbs, has since Sunday stepped up the right-wing rhetoric he has deployed during his campaign to try to woo FN voters.

His camp was defiantly ignoring the opinion polls -- one survey held after the first round said Hollande would beat Sarkozy by 54 percent to 46 in the second round.

"The right again believes it can do it," was the front-page headline of the pro-Sarkozy Le Figaro newspaper.

The incumbent has announced plans to hold a mass rally in Paris on May 1, the national labour day holiday on which the National Front every year holds a march in Paris to honour the memory of Joan of Arc.

Le Pen has said that at that march she will give her "opinion" to her supporters on whom to back in the decisive second vote, but analysts say it is unlikely she will endorse either candidate.

Gilbert Collard, head of the FN support committee, said Tuesday he believed "Marine Le Pen will probably give an instruction to vote blank".

Polls show that most far-right supporters prefer Sarkozy but up to a quarter -- mainly working-class voters attracted by Le Pen's protectionist trade policies -- could switch to Hollande.

Sarkozy repeated his promise after the first round to "protect the French way of life", tighten border controls, reduce immigration and promote a "buy European" policy.

All 10 candidates in the race, from the extreme right to the extreme left, railed against globalisation, financial markets and called for higher taxes on the wealthy.

Le Pen's score on Sunday was nearly double the 10.4 percent her father and former party leader, Jean-Marie Le Pen, got in 2007 when he defeated a Socialist in the first round before losing to Jacques Chirac in the second.

The French left has not won a presidential election in a quarter of a century, but with France mired in low growth and rising joblessness, opinion polls had long predicted Hollande would beat Sarkozy.

Hollande says Sarkozy has trapped France in a downward spiral of austerity and job losses, while Sarkozy says his rival is inexperienced and weak-willed and would spark financial panic through reckless spending pledges.

Hollande has already received the backing of other left-wing first round candidates, including Jean-Luc Melenchon who took just over 11 percent of the vote.