France was on high alert on Saturday, after an attacker stabbed a teacher to death in what the president called an act of "Islamist terror", prompting the deployment of thousands of troops and the temporary closure of renowned tourist sites.
Friday's attack took place in the northeastern town of Arras, home to large Jewish and Muslim populations.
A man in his twenties killed French teacher Dominique Bernard and seriously wounded three others at the school he used to attend.
On Saturday, amid fears the conflict between Israel and Hamas could lead to violence in foreign capitals, France announced it would deploy up to 7,000 soldiers under the its highest alert level.
Warnings of new attacks led to the evacuation of two of France's best-known monuments, the Louvre museum and Palace of Versailles.
Interior Minister Gerald Darmanin later said that following checks, there had been "no real threat" against the sites.
But since Hamas group's attack on Israel on October 7 there was "an atmosphere of jihadism".
Since then, 189 anti-Semitic acts have been reported in France, resulting in 65 arrests, 23 of whom were foreigners.
- 'Barbarity' -
Police arrested the suspected perpetrator of Friday's attack, Mohammed Moguchkov, who had cried the Arabic phrase "Allahu akbar!" (God is greatest).
He was among 11 people being held in custody on Saturday, a police source told AFP, including his 17-year-old brother and several other family members.
"This school was struck by the barbarity of religious terrorism," President Emmanuel Macron said after visiting the school.
The victim, Bernard, had "probably saved many lives" with his courage in blocking the attacker, he said.
Darmanin later said there was "probably a link between what's happening in the Middle East and this incident" in Arras.
The national anti-terrorist prosecutor announced it had opened an investigation.
French officials have said that Moguchkov was Russian, born in the predominantly Muslim North Caucasus republic of Ingushetia.
He was already on a French national register as a potential security threat and under electronic and physical surveillance by France's domestic intelligence agency, the DGSI, Darmanin said.
That was because of links with his father, who was also on the list and was deported in 2018.
His brother had also been imprisoned for an attack on the Elysee Palace in 2018, Darmanin said.
Moguchkov had been detained Thursday by the DGSI "to check whether he had a weapon" and to check his phone, Darmanin added. But he ruled out any "flaw in the intelligence services".
- 'Terrorism has struck' -
The teacher, Bernard, was stabbed in the throat and chest. Among those wounded were a school security guard who was stabbed multiple times and is fighting for his life, and a teacher in a less serious condition.
A cleaner was also hurt, according to anti-terror prosecutor Jean-Francois Ricard.
No pupils at the school were hurt.
School pupils throughout France will observe a minute's silence on Monday.
- Stepped-up protection -
Macron announced on Thursday that 582 religious and cultural facilities were receiving stepped-up police protection after the recent attack on Israel.
Speaking in Arras, he reaffirmed his message from that address for the French to "stand shoulder to shoulder" and "stay united".
Darmanin on Thursday banned pro-Palestinian demonstrations in France until further notice, as they "are likely to generate disturbances to public order".
In defiance of his order, several hundred people gathered in Paris and other French cities on Thursday shouting pro-Palestinian and anti-Israeli slogans, AFP correspondents said.
Police in Paris used tear gas to disperse the protesters, and said they had arrested 10 out of around 3,000 people present.