A teacher was killed and two other people severely wounded on Friday in a knife attack at a school in the town of Arras in northeastern France, police and regional officials said.
The attacker, who has not been named, was from Russia's mainly Muslim southern Caucasus region of Chechnya and was already on a French national register as a potential security threat, a police souce said.
Those wounded were a security agent who was stabbed multiple times and a teacher who is in a less serious condition, the source added.
No pupil at the school was hurt, said another police source.
The man has been detained by police, Interior Minister Gerald Darmanin wrote on X, formerly Twitter.
Local police said that the situation had been contained and no longer posed a danger to the public.
The school pupils and the teachers were confined to the school premises.
The perpetrator cried the Arabic phrase "Allahu akbar!" ("God is Greatest!"), according to the preliminary elements of the investigation, a police source who asked not to be named told AFP.
France immediately opened a terror investigation into the attack, National Anti-Terrorsim prosecutors said.
The country has suffered a series of attacks by Muslim perpetrators since 2015.
Most recently, the beheading of teacher Samuel Paty in broad daylight in 2020 near his school in a Paris suburb by a radicalized Chechen refugee led to a wave of shock and renewed debate about the influence of radical Islam.
The attack in Arras comes almost three years to the day after the murder of Paty which took place on October 16, 2020.
It also comes with tensions rising in France, which has large Jewish and Muslim communities, after last weekend's attack by Hamas on Israel.
President Emmanuel Macron said in an address to the nation on Thursday that 582 religious and cultural facilities in France were receiving stepped-up police protection.
"Those who confuse the Palestinian cause and the justification of terrorism commit a strong moral, political, and strategic error," he said.
His office said he would head to the scene in Arras.
There has also been controversy over the French government's ban on pro-Palestinian protests following the Hamas attack, with the left lamenting it was no longer possible to protest for peace but the right saying the measures did not go far enough.
Interior Minister Gerald Darmanin on Thursday ordered that the demonstrations be prohibited nationwide as they "are likely to generate disturbances to public order," adding that organizers should face arrest.
