An assailant on Friday decapitated a history teacher in France who had recently shown cartoons of the Prophet Mohammed in class, police said.
The attacker, whose identity has not been established, was shot by police as they tried to arrest him and later died of his injuries, they said.
French anti-terror prosecutors said they were treating the assault as "a murder linked to a terrorist organisation" and related to a "criminal association with terrorists".
Four people, including a minor, have been arrested, a judicial source told AFP early Saturday. All were related to the assailant, the source added.
French President Emmanuel Macron said the killing bore the hallmarks of "an Islamist terrorist attack", that "the entire nation" stands ready to defend teachers and that "obscurantism will not win".
The attack happened on the outskirts of Paris at around 5 pm local time near a school in Conflans Saint-Honorine, a northwestern suburb located some 30 kilometres from the centre of the French capital.
According to a police source, the victim was a history teacher at a local middle school who recently discussed cartoons of the Prophet Mohammed in class.
Attacker dies after shooting
Last month, charges were brought against a 25-year old Pakistani man who wounded two people with a meat cleaver to avenge the publication of caricatures of the Prophet Mohammed by the satirical weekly Charlie Hebdo.
The attacker seriously injured two employees of a TV production agency, whose offices are on the same block that used to house Charlie Hebdo. Both survived.
That attack came three weeks into an ongoing trial of suspected accomplices of the authors of the January 2015 attacks on Charlie Hebdo and a Jewish supermarket, which also saw a policewoman gunned down in the street.
Seventeen people were killed in the three-day spree that heralded a wave of Islamist violence in France that has so far claimed more than 250 lives.
The trial has sparked protests across France, with thousands of demonstrators rallying against Charlie Hebdo and the French government.
Police on Friday arrived at the scene after receiving a call about a suspicious individual loitering near the school, a police source said.
There they found the dead man and nearby sighted the suspect armed with a knife-like weapon, who threatened them as they tried to arrest him.
They opened fire and injured him severely, the source said. The man later died of his injuries, a judicial source said.
'Abominable'
The scene has been cordoned off and a bomb disposal unit dispatched because of the suspected presence of an explosive vest, the police source said.
Interior Minister Gerald Darmanin, who was on a visit to Morocco at the time of the attack, was to return to Paris immediately after talking with Mr Macron, as well as Prime Minister Jean Castex, his office said.
Mr Macron briefly joined key ministers at a crisis group set up in the interior ministry, the president's office said, before travelling to the scene of the attack.
French parliament suspended Friday's debate after news of the decapitation, with session president Hugue Renson, visibly moved, calling the attack "abominable".
MPs stood as Mr Renson said that "in the name of all of us, I want to honour the memory of the victim".
The attack comes only days after a follower of the Islamic State militant group who attacked a police officer outside Notre-Dame cathedral in Paris with a hammer was sentenced to 28 years in jail.
Farid Ikken, 43, charged at officers on patrol outside the cathedral on 6 June, 2017, shouting "this is for Syria".
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