The French prime minister has put the country on its highest state of counter-terrorism alert after an assailant fatally stabbed a teacher and seriously wounded two others.
Witnesses say the knifeman shouted “Allahu Akbar”, or “God is greatest”, during the attack at a school in Arras, northern France. He is now in custody.
The “attack emergency” level has been used in previous counter-terror cases.
The alert can trigger extra security deployments and public warnings.
On Saturday, 7,000 soldiers were mobilised for increased security patrols and the Louvre Museum in Paris was closed for security reasons.
Speaking to French news agency AFP, police said the Palace of Versailles had been evacuated on Saturday after a bomb threat.
The alert came via an anonymous message online, a source close to the matter told AFP adding that the palace, a major tourist attraction, would be closed at least for the rest of the day.
The attack at Gambetta high school in the northern city, at about 11:00 local time (09:00 GMT) on Friday, came amid rising tensions in France’s sizeable Muslim and Jewish communities, due to the conflict between Israel and Hamas.
Interior Minister Gérald Darmanin said there was “no doubt” a link between the Arras attack and the Israel-Hamas conflict.
The attacker, named as 20-year-old Russian national Mohamed Mogouchkov, is of Chechen origin and known to the security services for his involvement with Islamist extremism, according to police.
As a former pupil at the school, he alarmed teachers with his extremist language, reports say.
Police also arrested several members of the assailant’s family – a brother aged 17, his mother, a sister and an uncle.
Prime Minister Elisabeth Borne took the urgent step after a security meeting with President Emmanuel Macron.
Earlier Mr Macron visited the school and condemned the “barbarity of Islamist terrorism”. He called on French people to stay “united” in the face of the attack, to “not give in to terror or let anything divide us”.
The man killed was a French language teacher, stabbed in the throat and chest. Another teacher and a security guard were seriously wounded and are now in hospital. The security guard is critically ill, with multiple knife wounds.
A third person – a cleaner – was less seriously hurt in the attack, and no children were hurt.
Mr Macron said the teacher who died had “come forward to protect others and without doubt saved many lives”.
Martin Dousseau, a teacher who witnessed the attack, told AFP news agency of “a moment of panic during break-time, when the schoolchildren found themselves face-to-face with the armed man”.
“He attacked canteen staff. I wanted to go down to intervene, he turned to me, chased me and asked me if I was a history and geography teacher,” Mr Dousseau said. “We barricaded ourselves in, then the police arrived and immobilised him.”
The attack comes nearly three years since the murder and beheading of another teacher, Samuel Paty, at his school outside Paris.
The perpetrator of that attack, 18-year-old Abdullakh Anzorov, a Russian Muslim refugee, was shot dead by police shortly afterwards.
France has been hit by a series of Islamist attacks in recent years. The worst was in November 2015 when gunmen and suicide bombers attacked entertainment venues and cafes in Paris, killing 130 people. That attack was claimed by the Islamic State (IS) group.