People across France are expected to join rallies on Sunday in a show of solidarity and defiance following the beheading of a teacher outside his school for showing pupils cartoons of the Prophet Mohammed.
The killing of history teacher Samuel Paty in a Paris suburb on Friday has sparked outrage in France and memories of a wave of Islamist violence in 2015 sparked by caricatures of the Prophet Mohammed published by the satirical magazine Charlie Hebdo.
"It is absolutely important to show our mobilisation and our solidarity, our national cohesion," Education Minister Jean-Michel Blanquer told France 2, calling on "everyone (to) support the teachers".
One rally was set to take place at the Place de la Republique in Paris, a traditional site of protest where around 1.5 million people demonstrated in 2015 following a deadly attack on Charle Hebdo's office by Islamist gunmen.
Rallies were also expected in Lyon, Toulouse, Strasbourg, Nantes, Marseille, Lille and Bordeaux.
Paty had been the target of online threats for showing the cartoons, with the father of one schoolgirl launching an online call for "mobilisation" against him, France's anti-terror prosecutor Jean-Francois Ricard said.
The 18-year-old Chechen suspect, named as Abdullakh A, was shot dead by police shortly after the attack.
Witnesses said he was spotted at the school on Friday afternoon asking pupils where he could find Paty.
The schoolgirl's father and a known Islamist militant are among those arrested, along with several members of the suspect's family.
An 11th person was taken into custody on Sunday, a judicial source said.
Ricard said the school received threats after the class in early October, which featured the controversial caricatures - one of the prophet naked - with the girl's father accusing Paty of disseminating "pornography".
The aggrieved father named Paty and gave the school's address in a social media post just days before the beheading which President Emmanuel Macron has labelled an Islamist terror attack.
'Immersed in religion'
Ricard did not say if the attacker had any links to the school, pupils or parents, or had acted independently in response to the online campaign.
A photograph of Paty and a message confessing to his murder were found on the assailant's mobile phone.
The prosecutor said the attacker had been armed with a knife, an airgun and five canisters. He had fired shots at police and tried to stab them as they closed in on him.
He was in turn shot nine times, said Ricard.
The Russian embassy in Paris said the suspect's family had arrived in France from Chechnya when he was six and requested asylum.
Locals in the Normandy town of Evreux where the attacker lived described him as low key.
One resident who had been to school with him said he had become noticeably religious in recent years.
"Before, he got involved in fights but for the last two or three years he had calmed down" and had been "immersed in religion", he said.
Friday's attack was the second such incident since a trial started last month into the January 2015 massacre at the Paris office of Charlie Hebdo.
The magazine republished the cartoons in the run-up to the trial, and last month a young Pakistani man wounded two people with a meat cleaver outside the magazine's former office.
Ricard said Paty's murder illustrated "the very high-level terrorist threat" France still faces but added that the attacker himself was not known to French intelligence services.
An investigation is under way into "murder linked to a terrorist organisation".
The investigation will also look at a tweet from an account opened by the attacker, and since shut down, that showed a picture of Paty's head and described Macron as "the leader of the infidels".
Macron's office said a national tribute would be held for Paty on Wednesday.
On Saturday, hundreds of pupils, teachers and parents flooded to Paty's school to lay white roses.
Some carried placards stating: "I am a teacher" and "I am Samuel" - echoing the "I am Charlie" cry that travelled around the world after the 2015 Charlie Hebdo killings.
Martial, a 16-year-old pupil, said Paty had loved his job: "He really wanted to teach us things."
According to parents and teachers, Paty gave Muslim children the option to leave the classroom before he showed the cartoons, saying he did not want their feelings hurt.
Virginie, 15, said Paty showed the cartoons every year as part of a discussion about freedom following the Charlie Hebdo attack.