People commemorate victims of the terrorist attack against Charlie Hebdo in 2015 during a commemoration at the Place of the Republic in Paris, France, on Jan. 7, 2016.Photo: Xinhua

Tens of thousands of people Sunday rallied in Paris and cities across France in solidarity with a teacher beheaded for showing pupils cartoons of the Prophet Mohammed, as French President Emmanuel Macron promised swift action against online extremism.

Demonstrators on the Place de la Republique held aloft posters declaring: "No to totalitarianism of thought," and "I am a teacher" in memory of murdered colleague Samuel Paty.

"You do not scare us. We are not afraid. You will not divide us. We are France!" tweeted Prime Minister Jean Castex, who joined the Paris rally.

Castex was joined by Education Minister Jean-Michel Blanquer, Paris mayor Anne Hidalgo and junior interior minister Marlene Schiappa.

Some in the crowd chanted "I am Samuel," echoing the widespread "I am Charlie" cry after Islamist gunmen killed 12 people at the Charlie Hebdo satirical magazine in 2015 for publishing caricatures of the Islamic prophet.

Between bursts of applause, others recited: "Freedom of expression, freedom to teach."

"I am here as a teacher, as a mother, as a Frenchwoman and as a republican," said participant Virginie.

The Charlie Hebdo attack in 2015 unleashed a wave of Islamist violence and forced France into a national discussion about Islam's place in a secular society.

After the magazine assault, some 1.5 million people gathered on the same Place de la Republique in support of freedom of expression.

On Sunday, Macron ordered swift and "concrete action" to counter radical Islamist propaganda online.

At a meeting with six ministers and anti-terror prosecutor Jean-Francois Ricard, Macron said there would be "no respite for to those who organize to oppose the Republic's order."

The presidency said action would be taken against anyone expressing support for the attack, without giving any further details Sunday.

And authorities said from Monday, they would investigate the authors of 80 messages of support for the attacker.

Ministers in France's defense council also agreed to step up security at schools when they return after half term.

AFP