French investigators were Saturday probing the background of the Islamist militant who shot dead a policeman on Paris's prestigious Champs Elysees avenue as a jittery nation drew breath on the eve of the first round of a close-run presidential election.
Candidates have clashed over how to protect France since Thursday night's shooting, which the Islamic State (IS) group claimed as the work of one of its devotees.
Analysts say the 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.
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 world-famous boulevard rushing for cover.
The violent scenes thrust security to the fore of campaigning after nine months of relative calm. Le Pen, Fillon and Macron cancelled their final rallies Friday.
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.
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.
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.
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.
On Sunday, around 50,000 police and 7,000 soldiers will be deployed to protect voters.
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.
US President Donald Trump tweeted that the attack "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, because of his stature of statesman," Adelaide Zulfikarpasic of BVA pollsters said.
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".
Shop owners and restaurant managers shepherded their customers to backrooms and basements when the shooting began on the Champs Elysees.
"We heard the shots and people were running in every direction. But people were calm," said Lebanese tourist Zeina Bitar, 45, who was shopping with her children nearby.
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.