Firebrand leftist Jean-Luc Melenchon gathered thousands of supporters in Marseille on Saturday, on a weekend of rallies before the first round of France's presidential vote.

Communist-backed Melenchon, who has shaken up the vote with a surge in the polls, will be hoping for a repeat of rallies in Paris and Toulouse that were the biggest of the campaign, drawing tens of thousands onto the streets.

Sunday will then see the Socialist frontrunner, Francois Hollande, whose confidence was boosted this week as polls showed him cementing his lead, and incumbent right-winger President Nicolas Sarkozy hold rival Paris events.

Ahead of his beachfront rally, Melenchon joked on his blog that the two leading candidates were stealing a page from his playbook and "will now also occupy the streets and risk appearing in public squares".

The Left Front's Melenchon, who two polls Friday showed either tied or ahead of the far-right's Marine Le Pen in third place, has struck a chord with his virulent attacks on France's rich elite and EU-backed austerity measures.

On Sunday, Hollande and Sarkozy will be looking to recreate some of the fervour that has marked Melenchon's campaign, as they compete to draw crowds and media coverage at opposite sides of the capital

Sarkozy has called on supporters to gather in Paris's iconic Place de la Concorde while Hollande has summoned his backers to a concert outside the Chateau de Vincennes in working-class eastern Paris.

The first round of voting in a two-stage battle begins on April 22, and the race entered its final week with Hollande riding a wave of confidence.

He had seen his lead over right-wing Sarkozy narrow slightly in recent weeks, but the incumbent's late surge has not altered pollsters' predictions and the left remains on course for victory in the May 6 run-off.

Two polls published Friday by CSA and TNS-Sofres showed Hollande winning the first round with 27 or 28 percent of the vote, ahead of Sarkozy with 26 percent in both polls.

They predicted Hollande would also win a clear victory in the May 6 run-off vote, with 56 or 57 percent of the vote.

Other recent polls have shown Sarkozy ahead in the first round, but Hollande has enjoyed a consistent second-round lead in voting intentions since he won the Socialist primary in October and remains the clear favourite.

"We're going to win!" Hollande declared on Thursday night at a rally, only to correct himself the next morning when reporters asked about this newfound self-confidence: "We can win. There's your new slogan."

Sarkozy has lashed out at Hollande's tax-and-spend platform, warning that his victory would spark a crisis in investor confidence and a speculative attack on the euro that would bring France "to its knees".

Campaigning Friday in Corsica, Sarkozy urged voters to back him "for the future of the country" and warned that Hollande would govern only to please the left.

"A president of the republic must be the president of those who voted for him and of those who did not. That's the difference between me and Mr Hollande," Sarkozy said.

Analysts say 60-year-old former Socialist Melenchon's advance has kept predictions for Hollande's first-round score low but that most of his supporters are expected to swing to the centre-leftist before the run-off.

The two-week campaign between the two rounds is expected to be fierce, when the frontrunners go head-to-head and broadcasters will no longer be forced by law to accord equal time to outsider candidates.

Le Pen, new leader of her father Jean-Marie Le Pen's anti-immigrant, anti-EU National Front is also to hold a rally in Paris on Tuesday as she seeks to revive a campaign that has been overshadowed by Melenchon's surge.

And centrist Francois Bayrou, a one-time contender for third who is now stuck at about 10 percent in the polls, will hold a rally on Sunday in Marseille as he ends a campaign tour of major French cities.