Rallies are being held across France with thousands attending in support of Samuel Paty, the teacher beheaded after showing cartoons of the Prophet Muhammad to his pupils.
People in the Place de la République in Paris carried the slogan “Je suis enseignant” (I am a teacher), with PM Jean Castex saying: “We are France!”
A man named as Abdoulakh A was shot dead by police on Friday after killing Mr Paty close to his school near Paris.
An 11th person has now 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”.
The murder comes as a trial over the 2015 attack on Charlie Hebdo – a satirical magazine that has published cartoons of the Prophet Muhammad – is under way.
The Place de la République in Paris filled with people rallying in support of Mr Paty, 47. Mr Castex and Paris Mayor Anne Hidalgo joined them.
The square was the scene of a huge demonstration in which 1.5 million people showed solidarity with Charlie Hebdo following the deadly attack of January 2015.
One protester on Sunday carried a sign reading “zero tolerance to all enemies of the Republic”, another “I am a professor. I’m thinking of you, Samuel.”
Another told Le Figaro she was a French Muslim who was at the rally to express her disgust at the latest killing.
A minute’s silence was followed by the playing of the Marseillaise.
All the protesters were wearing masks to protect from coronavirus.
Mr Castex tweeted the rendition of the anthem, along with the words “you do not scare us… we are France!”