Paris (AFP) - The two frontrunners in the French presidential election, far-right leader Marine Le Pen and centrist Emmanuel Macron, staged rival rallies in Paris on Monday, seeking to stay ahead in a tightening race just days before the vote.

Macron staged his biggest rally of the campaign so far, attracting 20,000 people to the Bercy national indoor arena while a topless woman protester briefly interrupted Le Pen's speech to 6,000 supporters at a concert hall.

After weeks of twists and turns, the unpredictable race has narrowed dramatically, with surveys suggesting four candidates are in contention to win one of the top two spots in the vote next Sunday and progress to the run-off a fortnight later.

Scandal-hit conservative Francois Fillon and radical leftist Jean-Luc Melenchon are steaming up behind the two frontrunners, and with around one in four of the electorate still undecided, candidates are scrapping for every vote.

Macron and Le Pen, who have both scored as high as 25 percent in voter surveys, stood at 23 percent and 22.5 percent respectively in the latest Ifop poll updated daily, while Melenchon has surged to 19.5 percent, equal with Fillon.

Macron delivered an upbeat speech that focused on his vision of France in five years' time.

"We are going to turn the page on the last 20 years because our generation is ready for change," he told a crowd who chanted: "We're going to win."

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- 'We need the EU' -

The 39-year-old former Rothschild banker and economy minister gave a strong defence of the European Union in the face of attacks from Le Pen, who wants to withdraw from the bloc.

"We need Europe, so we will remake it," Macron told the crowd. "I will be the president of the awakening of our European ambitions."

His European Union would be "less bureaucratic" and would protect both "industrial and agricultural interests", he vowed.

In a reference to Le Pen, Macron said French voters had the choice of "hope and courage over resignation".

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The fast-growing score of Communist-backed Melenchon -- and the possibility he could square off against Le Pen in the May 7 decider -- has sparked alarm over the future of the EU as both advocate leaving the bloc.

Some observers predict that if Le Pen becomes president it could strike a mortal blow to the EU, already weakened by Brexit.

Dominique Dusart, 57, who heads Macron's En Marche (On the Move) movement in the Yonne area south of Paris, admitted supporters were worried his support could fall off in the crucial final days.

"We're a bit worried by Melenchon's breakthrough," she said. "It has been a bit of a slap in the face because we weren't expecting it."

Demonstrators briefly scuffled with police outside the Zenith hall in northeast Paris where Le Pen held her rally. And 15 minutes into her speech, a female protester burst onto the stage clutching a bunch of flowers with illegible slogans scrawled on her torso before she was bundled away by security guards.

"These left-wing extremists have got it all wrong," Le Pen said. "A total inversion of values has disrupted this speech by the only woman who defends women."

The National Front (FN) leader promised to tighten France's borders and crack down on immigration.

"Behind mass immigration, there is terrorism," she said.

- Melenchon's canal cruise -

Melenchon, 65, chose the quirkiest campaign event of the day, sailing through northeast Paris on a barge, making stops along the way to greet supporters.

Addressing supporters from the boat-deck, the leftist railed against the "fear mongering" of his rivals and the media about his big-spending programme and sympathies for the leaders of nations like Cuba and Venezuela.

"They make up things about us on a daily basis," he complained. "Keep the fire of rebellion burning inside you," he urged.

For his part Fillon, dogged by a fake jobs scandal that has seen him charged with abuse of public funds, said he was confident he would upend the polls.

"I can see things clearly. I am absolutely sure I'll be in the second round because there is a strong desire for change in our country and I am the only one proposing serious and reasonable change," the 63-year-old told reporters in the southern city of Nice before giving a speech to supporters.