Rallies are being held across France in support of Samuel Paty, a teacher who was beheaded after showing cartoons of Prophet Muhammad during a lesson.
Loud applause rose from the crowd in the Place de la Republique in Paris, with people carrying the slogan "Je suis enseignant" (I am a teacher), the BBC reported on Sunday.
A man named as Abdoulakh A was shot dead by police on Friday after killing Paty close to his school near Paris. An 11th person has been arrested as part of the investigation.
No details have been given about the arrest. Four close relatives of the suspect were detained shortly after the killing. Six more people were held on Saturday, including the father of a pupil at the school and a preacher described by French media as a radical Islamist.
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".
Samuel Paty, a well-liked teacher, had been threatened over showing the cartoons
In Lille, people carried banners and placards with the simple words "I am Samuel" and "I am a professor. I'm thinking of you, Samuel."
Thousands of people also gathered in Place Bellecour in Lyon to pay their respects, with another large turnout in Nantes.
Demonstrations are also being held in Toulouse, Strasbourg, Marseille, Bordeaux and elsewhere.
In addition to Sunday's demonstrations, there will be a national tribute paid to Paty, 47, on Wednesday.
"A civilisation does not kill an innocent person, barbarism does," Tareq Oubrou, imam of a mosque in Bordeaux, told France Inter on Saturday.
Laurent Nunez, the head of France's anti-terrorism task force, told France Info radio that a "line has been crossed". He condemned an act of "barbarity" that was an attack on "the freedom to teach".