Far-right presidential candidate Marine Le Pen said on Friday that France should immediately reinstate border checks and expel foreigners who are on the watch lists of intelligence services, adding that these were steps she would take, if elected.

Seizing on Thursday night's killing of a police officer in an attack claimed by Islamic State, Le Pen, who has been campaigning on a hardline anti-EU, anti-immigration platform, urged the Socialist government to carry out immediately measures that are included in her campaign manifesto.

"We cannot afford to lose this war. But for the past ten years, left-wing and right-wing governments have done everything they can for us to lose it. We need a presidency which acts and protects us," Le Pen told reporters at her campaign headquarters.

French voters elect a president in a two-round vote on April 23 and May 7. Opinion polls have for months forecast that Le Pen would make it through to the run-off, but then lose in the final vote.

Until now, Le Pen had struggled to get the campaign to focus on her party's trademark tough security and immigration stance. By contrast, she has been thrown on the defensive over her position to pull out of the euro zone, a proposal that lacks wide support.

Referring disparagingly to outgoing Socialist President Francois Hollande as "notoriously feeble", Le Pen said: "I only ask one last-ditch effort from him before leaving power: I solemnly ask him to effectively reinstate our borders."

She added: "Elected president of the Republic, I would immediately, and with no hesitation, carry out the battle plan against Islamist terrorism and against judicial laxity."

It is unclear what impact the attack will have on the first round of already very unpredictable presidential elections on Sunday.

With their hardline view on security and immigration, Le Pen and conservative Francois Fillon may resonate with some voters.

But other attacks that took place shortly before elections - the November 2015 attacks in Paris ahead of regional elections and the shooting in a Jewish school before the 2012 presidentials - did not have any effect on those ballots.