France was on edge 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 Sunday's 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.

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.

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 watchlist.

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

Fillon and 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 Fillon said, promising an ‘iron-fisted’ approach.

Macron, a 39-year-old moderate whom Fillon has portrayed as too inexperienced for the top job, said France was paying for the intelligence jobs cuts made when 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 Le Pen and Macron tied on 23 percent, ahead of Melenchon with 19.5 percent and Fillon on 19 percent.

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

Cheurfi drew up alongside a police van and shot an officer sitting at the wheel, sending shoppers and strollers on the ritzy Champs Elysees scattering for safety.

He was killed while trying to flee on foot. A German tourist was slightly wounded in the crossfire.

A statement by IS's propaganda agency Amaq issued shortly after the attack identified the assailant as ‘Abu Yussef the Belgian’.

The claim had raised concerns that a possible second attacker could be on the loose.

French authorities said a man sought in Belgium, who was suspected of having planned to travel to France on Thursday, had handed himself in to police in the Belgian city of Antwerp.

Cheurfi was arrested in February on suspicion of plotting to kill police officers but released because of a lack of evidence.

A serial offender, he spent nearly 14 years in prison for a range of crimes including attacks on the police. He had shown ‘no signs of radicalisation’ while in custody, said France's anti-terrorism prosecutor Francois Molins.


- '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.

US 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 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.

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