The teenager who beheaded a teacher outside a Paris school posted a photograph of the body on Twitter before he was shot dead by police.

Anti-terrorism prosecutor Jean-Francois Ricard said on Saturday that the photograph was accompanied by a message claiming responsibility for the attack, and was discovered on the assailant's phone found near his body.

The post was removed swiftly by Twitter, which said it had suspended the account because it violated the company's policy.

Ricard revealed the message said "In the name of Allah the most gracious, the most merciful, ... to (President Emmanuel) Macron, leader of the infidels, I have executed one of your hell-hounds who dared to belittle (Prophet) Mohammad."

The 18-year-old, who was born in Russia, was shot dead by French police minutes after he murdered 47-year-old history teacher Samuel Paty in broad daylight outside the school where he taught, in Conflans-Sainte-Honorine on Friday.

The attacker, of Chechen origin, had been living in the town of Evreux northwest of Paris, and was not previously known to the intelligence services, Ricard said.

Ricard also told the news conference that the attacker had approached pupils in the street before the incident and asked them to point out his victim.

Earlier this month, Paty had shown his pupils cartoons of the Prophet Mohammad in a class on freedom of expression, upsetting some of the Muslim parents. Many Muslims feel that any depiction of the Prophet violates their religious beliefs.

Muslim leaders condemned Friday's killing, which many public figures perceived as an attack on French values.

Any deadly attack by Islamist militants or their sympathizers was devastating for France's Muslim community, Tareq Oubrou, the imam of a Bordeaux mosque, said.

"We are between hammer and anvil," he told France Inter radio. "It attacks the Republic, society, peace and the very essence of religion, which is about togetherness."

The killing shocked the country and carried echoes of an attack five years ago on the offices of satirical magazine Charlie Hebdo, which published caricatures of the Prophet Mohammad that many Muslims found offensive.

Unions, anti-racism groups and Charlie Hebdo organized a gathering in central Paris on Sunday to commemorate the murdered teacher.

A national tribute will be organized for Wednesday, president Macron's office said.

In an outpouring of solidarity, the hashtag #JeSuisSamuel (I am Samuel) trended on social media, like the #JeSuisCharlie call for solidarity after the attack on Charlie Hebdo in 2015.

Four close relatives of the attacker were detained soon after the attack. Five more were detained overnight, including the father of a pupil at Paty's school and an acquaintance of the pupil's father known to the intelligence services, the anti-terrorism prosecutor said.

However, it was not immediately clear if the teenage attacker knew either the pupil's father or the father's acquaintance.