MARSEILLE — French far-right candidate Marine Le Pen on Wednesday accused her rivals of turning a blind eye to Islamic terrorism at her last major rally before the first round of a nailbiting race for president.

Addressing nearly 5,000 flag-waving supporters in the southern city of Marseille, where two Frenchmen were arrested Tuesday on suspicion of planning an attack on the election, the National Front (FN) leader said: "I have been denouncing this terrible poison of Islamic terrorism since I launched my campaign ... and none of my rivals are willing to debate the subject.

"They wanted to stay quiet about this problem, to suppress it, to keep it at a distance like one sweeps dust under the carpet," she said.

"The silence of the other candidates comes from shame, the shame of being a member or even having led a government which did nothing to lower the threat and even created the conditions for this scourge to develop," she told the rally, which was held under tight security.

Police fired teargas and arrested four people after around 500 people demonstrated against Le Pen and tried to march towards the rally. Some of them threw projectiles and firecrackers.

"Terrorist acts have multiplied throughout Europe during the campaign," she said. "Here in Marseille two fanatics were arrested before they committed odious crimes."

Le Pen, 48, has spent years trying to grow support for the FN by campaigning on bread-and-butter issues, but in the final days of the race she has returned to its stock themes of immigration and national identity.

In a BFM television interview on Wednesday she repeated that she would slash immigration, make it harder to get French nationality and crack down on suspected Islamists.

Polls show Le Pen, 39-year-old centrist Emmanuel Macron, conservative former prime minister Francois Fillon and hard left firebrand Jean-Luc Melenchon locked in a tight four-way race ahead of Sunday’s first round of voting.

A new survey published Wednesday showed Macron leading on 23 per cent to Le Pen’s 22.5 percent -- both of them down slightly.

Fillon had improved to 19.5 per cent while Melenchon had reached 19 per cent, narrowing the gap with the frontrunners.

The top two will advance to a run-off vote on May 7.

Jean-Yves Camus, an academic and expert on the FN, said that Le Pen had returned to the "fundamentals, and particularly immigration" in the final run-in.

"It’s back to basics," he said. "The FN top brass has decided to go full throttle for far-right voters."

Struggling Socialist candidate Benoit Hamon, whom polls place fifth, attempted to turn around his campaign at a rally in Paris.

Hamon urged voters to "wake up, because you alone can avoid a nightmare for France." — AFP