The gunman, identified as Karim Cheurfi, opened fire on a police vehicle parked on the Champs Elysees in Paris late on Thursday, killing one officer and injuring two others before being shot dead.
Police found a note defending Islamic State near his body and believe he had "opened fire on the officers in the knowledge he would be killed by them", a source close to the investigation said.
Coming just days after police said they had foiled another planned Islamist attack, arresting two men in the southern city of Marseille, the Champs Elysees shooting dominated the final day of election campaigning.
Pierre Henry Brandet, of the French Interior Ministry, said the attacker stopped his auto in front of a police van and emerged with an automatic weapon.
Two other police officers were killed in the attack, which ended when cops shot and killed the suspect.
The attacker was targeting police near the Franklin Roosevelt subway station, a police spokeswoman told the Associated Press.
"Nothing must hamper this democratic moment, essential for our country", Prime Minister Bernard Cazeneuve said after a high-level meeting Friday that reviewed the government's already heightened security plans for the two-round presidential vote that begins Sunday. ISIS claimed responsibility for the attack and gave a pseudonym for the shooter, Abu Yusuf al-Beljiki, indicating ties to Belgian or had lived in Belgium.
Fillon also said the fight against "Islamist totalitarianism" should be the priority of the next president.
ISIS claimed that the attack was carried out by one of its "fighters", whom it called "the Belgian".
Police check passersby near the Champs Elysees in Paris after a shooting which left one officer dead and two wounded.
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French President Francois Hollande (right) is surrounded by ministers and officials as they hold a defense council at the Elysee Palace in Paris, April 21, 2017.
Bloodshed had always been feared ahead of Sunday's first round of voting after a string of jihadist atrocities since 2015, and the shooting on the world-renowned boulevard forced security to the top of the agenda in the campaign.
Steve White, chairman of the Police Federation of England and Wales, said: "Once again the police family is in mourning and it serves to underline how courageous officers are putting their life on the line day in and day out to protect the public".
The killer identified by French authorities was known to anti-terror police, sources told AFP.
"It looks like another terrorist attack".
A foreign tourist was slightly wounded in her knee by shrapnel during the shooting.
People on the French security services' watch list for radicalization should also be expelled from France and have their French citizenship taken away, she said.
Police are urging the public to avoid the popular tourist area. Archive reports by French newspaper Le Parisien say that Cheurfi was convicted of attacking a police officer in 2001.
"Wait and see attitude is likely to prevail in the last trading day before the first round of the French elections; with yesterday's events in Paris adding to uncertainty", chipped in analysts at Unicredit.
Francois Fillon, who is the conservative candidate, said he would cancel the campaign events had had been planning for Friday as a result of Thursday's attack.