PARIS (AP) — Extremism concerns shook France’s presidential campaign Tuesday as authorities announced arrests in what they said was a thwarted attack and candidates urged tougher counterterrorism efforts for a country already under a state of emergency.

While national security previously has been a strong theme in the campaign, right-wing candidate Marine Le Pen hardened her tone on foreign extremists and border controls in the wake of the arrests that came days before the first round of voting.

Centrist Emmanuel Macron called for national unity and stronger intelligence. Le Pen and Macron are among four leading candidates seen as most likely to progress from Sunday’s first round and to reach the May 7 runoff between the top two.

As the government prepared to flood streets with more than 50,000 police and soldiers to safeguard the ballot, Interior Minister Matthias Fekl said police thwarted an imminent “terror attack,” arresting two French men in the southern port city of Marseille.

Both are suspected Islamic radicals, according to Paris prosecutor Francois Molins. Police seized guns and explosives from the apartment the men were leaving when they were arrested, Molins said. The explosives found are of a type used in previous attacks in France and Belgium that were inspired by the Islamic State group.

It was unclear whether a campaign event was a potential target for the attack; Molins said investigators have not determined “the day, the targets and the exact circumstances” of the suspects’ plans.

Macron’s campaign team said authorities earlier provided a photo of the suspects to his security detail.

The presidential election is being watched as a bellwether for global populist sentiment, in large part because of Le Pen’s nationalist, anti-immigration positions.

In a written statement Tuesday, Le Pen pointed to “a devastating multiplication of attacks and threats of attacks” in France which she said was the result of “Islamic fundamentalism” that “has expanded exponentially” in the last decade in the country.

“It’s time to put back France in order,” she said, using one of her campaign’s mantras.

Before Tuesday’s arrests were announced, Le Pen said on RTL radio that she would expel foreign extremists and draft army reservists to close France’s borders as soon as she takes office.

“We cannot fight the terrorism that weighs on our country without controlling our borders,” she said.

Macron struck a tough, but conciliatory tone.

He called the arrests a reminder that “the terrorist threat remains very high,” especially during the election campaign, and reiterated calls for pressure on internet companies to better monitor extremism online.

But he added that “terrorism … is a challenge that calls upon us more than anything else to come together, because the terrorists wish nothing more than our division.”

Macron and conservative candidate Francois Fillon have pledged more robust counterterrorism efforts, but remain committed to Europe’s open borders.

“Democracy must not get on its knees in front of the threats and intimidations from terrorists,” Fillon said in a written statement. “The campaign must continue until the end.”

Far-left candidate Jean-Luc Melenchon suggested his three main rivals – Fillon, Le Pen and Macron – could have been potential targets of the two suspects in Marseille. He expressed solidarity with his fellow presidential hopefuls.

“We will never make the gift to criminals to divide in front of them. We are not afraid,” Melenchon said during a rally in Dijon.

Melenchon, who leads a far-left alliance that includes the Communist Party, has risen in polls in recent weeks and is now considered as having a chance of reaching the runoff election.

France’s fight against homegrown and overseas Islamic extremism has, with jobs and the economy, been one of the main issues for the stumping presidential candidates.

Those on the right have been particularly vocal, seeking to appeal to voters traumatized by IS-inspired attacks that have killed at least 235 people in France since January 2015, by far the largest casualty rate of any Western country.

With the terror threat “higher than ever,” Fekl said “everything is being done” to secure the election, the candidates, their election headquarters and rallies.

He said more than 50,000 police, gendarmes and soldiers will be deployed in France and its overseas territories on Sunday and during the decisive May 7 second round.