A teacher has been killed in a knife attack in a school in the northern France city of Arras and the investigation handed to the anti-terrorism prosecutor's office.
President Emmanuel Macron condemned the attack as the "barbarity of Islamist terrorism".
The regional Pas-de-Calais authority said the suspected assailant, who also wounded a second teacher and a school security guard in the attack, was arrested.
The suspect was a Russian-born Chechen and former student of the Lycee Gambetta high school where the attack happened, a police source said.
He was on a watchlist of people known as a potential security risk in connection to radical Islamism, the police source added.
Police could not confirm local media reports that he shouted "Allahu Akbar". BFM TV reported he was about 20 years old.
France this evening raised its alert level following the attack, the office of French Prime Minister Elisabeth Borne told AFP.
The level was upped after a security meeting attended by President Macron, the office said.
"We're all in a state of shock," said philosophy teacher Martin Doussaut, who was chased down by the attacker, but managed to escape unharmed after locking himself down in a room.
BFM TV also said the person killed was a French language teacher, while a sports teacher was stabbed and injured.
Pupils were confined to their classrooms, it added.
President Macron ias heading to Arras, his office said. A planned meetign with Taoiseach Leo Varadkar has been postponed.
He said that a second attack had been prevented in another part of France.
The teacher had "undoubtedly saved many lives" in facing down the attacker, Mr Macron said, calling on France to stay "united" in the face of the attack.
He did not specify the location of the second attempted attack.
In a televised address to the nation yesterday, he urged the French to remain united and refrain from bringing the Israel-Hamas conflict home.
Nothing so far pointed to a link with events in Israel and Gaza, a second police source told the news agency Reuters.
The suspected assailant's brother was also arrested.
La Voix du Nord newspaper said that pupils in all schools in Arras - a town in the ethnically diverse northern corner of France - were held in their classrooms for their own safety.
France has been targeted by series of Islamist attacks over the years, the worst being a simultaneous assault by gunmen and suicide bombers on entertainment venues and cafes in Paris in November 2015.
