France was on edge on Saturday on the eve of its most unpredictable presidential election in decades, which will take place under heightened security after the jihadist killing of a policeman.

The Islamic State-claimed slaying of the officer on Paris’ Champs Elysees avenue thrust questions of security to the fore of campaigning after nine months of relative calm.

Analysts say Thursday’s attack could shake up the four-way contest between far-right leader Marine Le Pen, centrist Emmanuel Macron, conservative Francois Fillon and Communist-backed firebrand Jean-Luc Melenchon.

The top two vote getters in the April 23, 2017 poll will head to a run-off on May 7.

Authorities in Paris have offered additional guards for hundreds of polling stations in the capital, which will come on top of an already major security plan across the country.

“An extra guard or reinforcement of staff will be provided to any polling station that needs it,” Paris town hall official Colombe Brossel said.

On Sunday, around 50,000 police and 7,000 soldiers will be deployed to protect voters across France.

Until now, surveys showed the French to be more concerned about jobs and the economy than terrorism or security, though analysts warned Thursday’s shooting could change that.

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A note praising IS was found next to the body of 39-year-old gunman Karim Cheurfi, who shot dead an officer and wounded two others before being killed in a firefight that sent tourists on the Champs Elysees rushing for cover.

Ms. Le Pen has moved quickly to present herself as the strongest defender against Islamist radicals in a country scarred by a string of attacks that have claimed 239 lives since 2015.

‘No signs of radicalisation’

The 48-year-old leader of the anti-immigration National Front (FN) called for France to “immediately” take back control of its borders from the European Union and deport all foreigners on a terror watch list.

“This war against us is ceaseless and merciless,” she said, accusing the Socialist government of a “cowardly” response to the threat.

Mr. Fillon and Mr. Macron also hastily convened televised briefings in which they vowed to protect the country.

“Some haven’t taken the full measure of the evil,” 63-year-old Mr. Fillon said, promising an “iron-fisted” approach.

Mr. Macron, a 39-year-old moderate whom Mr. Fillon has portrayed as too inexperienced for the top job, said France was paying for the intelligence jobs cuts made when Mr. Fillon was Prime Minister between 2007 and 2012.

Describing the Champs Elysees shooting as an attack on democracy, he urged voters: “Do not give in to fear.”

Veteran left-winger Melenchon, 65, was the only one of the four to stick to his schedule.

A BVA poll conducted on Thursday and Friday showed Ms. Le Pen and Mr. Macron tied on 23%, ahead of Mr. Melenchon with 19.5% and Mr. Fillon on 19%.

Though the race has four main contenders, a total of 11 are in the race, most of whom are polling in the single digits.

‘Exploiting’ attack

The shooting came days after two men were arrested in Marseille on suspicion of planning an imminent attack and follows a series of deadly strikes around Europe in the past month, targeting Stockholm, London and the Saint Petersburg metro.

U.S. President Donald Trump tweeted that the attack “will have a big effect” on the election.

Adelaide Zulfikarpasic of BVA pollsters said: “If it were to benefit someone that would clearly be Marine Le Pen who has dominated this issue throughout the campaign, or Francois Fillon.”

Prime Minister Bernard Cazeneuve accused Ms. Le Pen of attempting to make political hay out of the killing, saying she was “seeking, as she does after every tragedy, to take advantage of it”.

France has been under a state of emergency for nearly a year and a half.

The string of terror attacks began in January 2015 with a massacre at the offices of the Charlie Hebdo satirical magazine.

The following November, IS gunmen and suicide bombers killed 130 people in Paris, and a Tunisian man rammed a truck through crowds in Nice last July, killing 86 people.

How does the election work?

Nearly 46.87 million voters are registered to vote. The country’s 66,546 polling stations will open at 8 a.m. local time (11.30 a.m. IST) and close at 7 p.m. (10.30 p.m. IST), an hour later than the last election in 2012.

In Paris and other big cities, the polls remain open until 8 p.m. (11.30 p.m. IST).

The election is the first in the history of France’s 59-year-old Fifth Republic to take place under a state of emergency.

Over 50,000 police backed by 7,000 soldiers from the Sentinelle (Sentry) anti-terror operation will be on patrol during the vote.

When will we get the results?

Projections based on partial results usually come in at 8 p.m. (11.30 p.m. IST) but could be delayed due to the extra hour of voting.

The two top will go through to a run-off on May 7.

The next President will be sworn in by May 14 at the latest, taking over from Socialist President Francois Hollande.