The teacher who was beheaded in a street in France had received threats after showing controversial cartoons of the Prophet Muhammad to his pupils, French media report.
He has been named as 47-year-old Samuel Paty, a history and geography teacher.
Nine people have been arrested, including the parents of a child at Mr Paty’s school, judicial sources are quoted as saying.
Police say the attacker was an 18-year-old man of Chechen origin.
The killing took place while a trial is under way in Paris over a 2015 Islamist assault on the French satirical magazine Charlie Hebdo, which was targeted for publishing the cartoons.
President Emmanuel Macron said the attack bore all the hallmarks of an “Islamist terrorist attack” and the teacher had been murdered because he “taught freedom of expression”.
Speaking at the scene hours after the incident, he stressed national unity. “They will not prevail, they will not divide us,” he said.