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, reports BBC News.
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.