Paris (AFP) - 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 shooting on Paris's world-renowned Champs Elysees avenue on Thursday, claimed by the Islamic State group, thrust questions of security to the fore of campaigning after nine months of relative calm.

On Saturday, police arrested a man carrying a knife at Paris's Gare du Nord station, briefly causing panic as some passengers rushed out of the way.

France goes to the polls on Sunday in an election seen as crucial for the future of a deeply divided country and the beleaguered European Union.

Surveys suggest that nearly a quarter of voters are still undecided and that until now the French have been more concerned about jobs and the economy than terrorism.

But analysts said the policeman's shooting could shift opinions, perhaps handing an advantage to candidates seen as taking a hard line on security, such as far-right leader Marine Le Pen.

- 'Don't give in to fear' -

Le Pen and 39-year-old centrist former banker Emmanuel Macron are leading the polls but the race has tightened in the final days and any one of four candidates could reach the runoff on May 7.

Conservative Francois Fillon and hard-left firebrand Jean-Luc Melenchon are both just a few points behind the frontrunners.

Around 50,000 police and 7,000 soldiers will be deployed to protect voters on Sunday, with the contingent boosted in Paris after the Champs Elysees shooting.

The killing of policeman Xavier Jugele by 39-year-old gunman Karim Cheurfi was the latest in a string of terror attacks in France since 2015 that have claimed more than 230 lives.

US President Donald Trump tweeted that he thought the shooting "will have a big effect" on the election.

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

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Le Pen moved quickly after the shooting to present herself as the strongest defender against Islamist extremists, calling for France to "immediately" take back control of its borders from the EU and deport all foreigners on a terror watchlist.

Socialist Prime Minister Bernard Cazeneuve accused Le Pen of attempting to exploit the killing, "as she does after every tragedy".

Macron described the shooting as an attack on democracy, urging voters: "Do not give in to fear."

Voting began Saturday in several overseas French territories such as Martinique and Guadeloupe in the Caribbean, and French citizens cast ballots in the US.

Adrien Gontier, an expat voting at the French embassy in Washington, said he was fulfilling his duty as a citizen.

"In the United States, you can see what happens when people don't vote, or vote badly," he said. "We don't want there to be a Trump in France."

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.

A total of 11 candidates are in the running, most of them in single digits.

- 14 years in prison -

A clearer picture was meanwhile emerging of the Champs Elysees gunman's violent past.

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 police. He had shown "no signs of radicalisation" while in custody, said France's anti-terrorism prosecutor Francois Molins.

Three of his close associates were released after questioning late Saturday.

"Their interviews notably allowed us to gather certain elements on the attacker's profile and personality," a source close to the inquiry said.

Cheurfi's former lawyer Jean-Laurent Panier described him as "very solitary and introverted".

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 Saint Petersburg.

By sad coincidence, slain policeman Jugele had gone to the Bataclan concert hall -- one of the targets in November 2015's Paris attacks -- on the night of its reopening, as an ordinary concert-goer.

"I am here with my friend to celebrate life and to say no to terrorism," he had said in a BBC radio interview that emerged Saturday.

Some 200 angry wives of police officers held a protest near the Eiffel Tower on Saturday, demanding better protection for their spouses while on duty.