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.
The attack at Gambetta high school in the northern city, at about 11:00 local time (09:00 GMT), 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.