Paris: Nicolas Sarkozy and Socialist rival Francois Hollande on Tuesday stepped up their battle 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," Hollande told a 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 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 today 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 has since Monday stepped up the right-wing rhetoric he has deployed during his campaign to try to woo FN voters.

At a rally in Longjumeau in the Paris suburbs on Tuesday, he hammered home the themes he has pushed for months -- protecting "the French way of life," reducing immigration, and encouraging hard work.

He said the FN's first-round result, which shocked many in France and raised concerns across Europe, was not "reprehensible" because Le Pen had the right to stand for office and was therefore "compatible with the republic."

"The FN vote must be understood," he told supporters.

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