France's increasingly fraught presidential race is to overshadow May Day celebrations Tuesday, with right-wing incumbent Nicolas Sarkozy staging a rally on a day traditionally dominated by unions.

Less than a week before Sunday's run-off pitting him against Socialist favourite Francois Hollande, Sarkozy will address a campaign meeting at the Trocadero in Paris's chic 16th arrondissement to speak about work "values".

And anti-immigrant presidential candidate Marine Le Pen, knocked out in the April 22 first round after scoring a record 18 percent of votes, is to hold her own rally in Paris in memory of far-right icon Joan of Arc.

At the same time, unions will march to the capital's Place de la Bastille, but without Hollande, who will be attending a memorial in central France for former prime minister Pierre Beregovoy who killed himself in 1993.

Instead, Socialist Party secretary Martine Aubry will speak at the mass Bastille rally, while Sarkozy's UMP party hopes that tens of thousands will turn out at the Trocadero meeting.

Sarkozy initially called his rally in the name of "real work" before deciding to focus on the exposure of private sector workers to the economic crisis, in contrast with public sector workers who are "blocking France".

On Monday he fired off a fresh salvo at the CGT union and its leader Bernard Thibault for "betraying the cause of unionism" after Thibault called for a vote against him.

"CGT leaders, notably Mr Thibault, are members of the Communist Party, at least things are clear," said Sarkozy, who says Hollande will be "behind the CGT's red flags" on May Day while he himself addresses "a forest of tricolour flags."

CFDT union leader Francois Chereque said on Monday that it was a mistake for the CGT to have called to vote against Sarkozy, but said he had "never heard such violent words against unions."

The National Front's rally, an annual event, will climax at the Opera in Paris where Marine Le Pen is to address her supporters having achieved a record score for her party just over a year after taking over from her father, Jean-Marie Le Pen.

Five years ago, party founder Le Pen senior called at the same spot for his supporters to cast their vote for neither Sarkozy nor Socialist Segolene Royal in the run-off.

Like her father in 2007, Marine Le Pen is expected to call for her supporters to "save their votes" for parliamentary elections in June and to attack Sarkozy for having "stolen" FN votes with his far-right rhetoric.

With campaigning to end on Friday, the sole televised debate between Sarkozy and Hollande on Wednesday is widely perceived as the right-winger's last chance to take votes away from the Socialist.

Government spokeswoman and Budget Minister Valerie Pecresse said the debate could change things, despite Sarkozy trailing Hollande in opinion polls for months, the latest predicting a Socialist win by 53 to 47 percent.

"We're in the last week and today the question is about credibility, not popularity," Pecresse told Radio Classique, adding that "none of Hollande's promises during this campaign has financing, none can be realised."

Hollande's campaign chief Pierre Moscovici said he hoped for the Socialist to win the debate, to be watched by tens of millions of French, but noted that historically the head-to-head has not had much impact on final results.

"Wednesday's duel must be dignified, but also a debate that Francois Hollande wins," Moscovici told RMC radio.

"The debate has so far never turned an election," he said.

"This confrontation, that we owe to the French, is something they expect, it's more than a habit, it's a powerful moment in a presidential race."