The killing of a teacher at a school in France was a terror attack, Emmanuel Macron has said.
The president added that police had helped thwart another attempted attack elsewhere in France today.
The teacher was stabbed to death at the Lycee Gambetta secondary school in Arras, in the north of the country.
Two other people, including another teacher and a security guard, were seriously injured and were fighting for their lives in hospital following the knife attack.
A suspect, believed to be a former student at the school, has been arrested. One of his brothers was also detained nearby.
The suspect was reported to be in his 20s, Russian-born and of Chechen origin, and was on a watchlist of people known to be at risk of radicalisation.
He had been under surveillance since the summer and was stopped as recently as Thursday for a police check which found no wrongdoing, the French intelligence services said.
Teacher ‘saved many lives’
“The teacher who was killed had come forward to protect others and had without doubt saved many lives,” said Mr Macron after visiting the school.
He called the attack an act of “barbaric Islamist terrorism” and said the school would reopen on Saturday, adding: “Our choice is made not to give in to terror, not to let anything divide us.”
The victim has been named by local media as Dominique Bernard, a French language teacher.
Mr Macron did not give more details of the second attempted attack, but police said a man armed with a knife was arrested coming out of a prayer hall in the Yvelines region west of Paris. The man’s motives were not immediately clear.
A police officer, who was one of the first on the scene of Friday’s attack in Arras, said the suspect shouted “Allahu Akbar” – which means “God is great” in Arabic – before the stabbing.
Residents have been advised by local authorities to avoid the centre of the city, which is about 30 miles south of Lille.
Students were reportedly locked down in the school during the incident.
None of the children were physically harmed during the attack, according to reports.
Arras is shocked and bewildered and wants answers
In the heart of Arras, not far from a pedestrianised shopping precinct, there are hundreds of armed police officers.
French President Emmanuel Macron has come to town, along with two of his most trusted lieutenants – the interior and education ministers.
But the atmosphere is one of shock and sadness.
As I arrive, guided through a police cordon, I see a man walking away, his arm draped around his subdued teenage daughter.
The Lycee Gambetta stands ahead of us. It is a forgettable building, softened by tall trees. But now, it is surrounded by police vans and incident tape.
What happened at the school was horrendous – a knife attack of particular savagery that has shaken people here.
I spoke to one student outside the school, a thoughtful sixth-former called Remi.
He told me Arras was a quiet, safe town. “I’d say it was chilled,” he said – and that he had been shocked when he heard the news of the attack: “Why would you do something like that? Why would have so little value for a human life?”
The question is why?
Why did this man do something so brutal? Was it an isolated incident, was it inspired by the conflict in the Middle East, or by the ongoing resonance of the murder of Samuel Paty, almost exactly three years ago.
Was he motivated by Islamist fury, or by some other grievance. Arras, like the French nation, wants answers but at the moment, this town reverberates simply to shock, bewilderment and sadness.
‘We ran’
Local media quoted one pupil as saying: “We came out of class to go to the canteen, and we saw the guy with two knives attacking the teacher, who had blood on him.
“He tried to calm him down and protect us. He told us to get out, but we didn’t understand. We ran, and others went back upstairs.”
A security alert was sparked later at another school in Arras.
A third man was reportedly arrested in that incident, when he tried to enter the school with a suspicious rucksack.
Education Minister Gabriel Attal has urged schools across France to “immediately take all measures” to increase security.
Naima Moutchou, a vice president of France’s National Assembly, expressed “solidarity and thoughts for the victims, their families and the educational community” on behalf of the assembly’s representatives.