Police shot and killed Cheurfi after he opened fire on a police van on Paris' most famous boulevard.
Several Femen activists were arrested after staging a topless protest against Le Pen meters from the polling station where the far-right leader was heading to vote. But Belgium's interior minister said the pseudonym did not belong to the attacker.
The extremist group claimed responsibility for the attack in an unusually quick statement. Police found a note defending ISIS ideology next to Cheurfi's body.
Sunday's vote is the first round in the French elections, with the top two candidates advancing to a winner-takes-all runoff on May 7.
In an interview with The Associated Press, Trump says he is not explicitly endorsing Le Pen.
Municipal workers in white hygiene suits were out before dawn to wash down the sidewalk where the assault took place - a scene now depressingly familiar after multiple attacks that have killed more than 230 people in France over two years. Bernard Cazeneuve, the Socialist prime minister, accused Le Pen of electoral opportunism in the face of a tragedy. "This must be the priority", he said. He also called for the creation of 10,000 more police posts.
"We don't understand why Islamic State has identified the wrong person", said a police source.
In November a year ago, according to L'Express newspaper, he had attended a concert that reopened the Bataclan Theater, the main target in a series of Islamic State attacks on Paris in November 2015. We are at war.
Surveys suggest that almost a quarter of voters are still undecided and that until now the French have been more concerned about jobs and the economy than terrorism.
Cazeneuve, however, questioned Fillon's position on security, saying that when he previously served as Prime Minister he had cut thousands of security force jobs.
Macron described the shooting as an attack on democracy, urging voters: "Do not give in to fear".
"I'm not going to let myself be influenced by people who are trying to frighten us", Paris resident Anne-Marie Redouin said in front of a military patrol near the heavily-guarded Eiffel Tower.
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Alternatively, if neither candidate makes it past Sunday's first round into the May 7 runoff, that's a clear message that populist nationalism is receding.
The killing of a policeman by a suspected Islamist militant pushed national security to the top of the French political agenda on Friday, two days before the presidential election.
French presidential candidate Jean-Luc Melenchon at a campaign rally in Lille, April 12, 2017.
Brandet later told BFM TV that a man with that name had turned himself in at a police station in Antwerp and was no longer being searched for in France.
The gunman was then shot dead as he tried to escape, Paris prosecutor François Molins told reporters.
"What our attackers want is death, symbolism, to sow panic (and) to disturb a democratic process", he said. He was jailed again in 2013 for lesser offenses and released on probation two years later. This morning, authorities published that Cheurfi was arrested only 2 months ago, as part of a counter-terrorism investigation and a knife and mask were found in his possession.
"He was not on the security watch list and had shown no signs of radicalization", stated Molins.
As with all other similar attacks in France, investigators are looking into the possibility of accomplices.
Political campaigning was banned from midnight Friday hours ahead of polls opening in France's far-flung overseas territories such as Guadeloupe, French Polynesia and French Guiana, which all voted a day early Saturday.
President Donald Trump says businesses and individuals will receive a "massive tax cut" under a tax reform package he plans to unveil next week.
Fillon separately pledged to maintain the state of emergency that has been in place since IS-claimed gun and bomb attacks killed 130 people in Paris in November 2015. A day after a terror attack in Paris that left one dead and three wounded, Trump took to the social media site to weigh in.
When the French vote for president Sunday, their choice will resonate far beyond France's borders, from Syrian battlefields to Hong Kong trading floors and the halls of the U.N. Security Council.