PARIS • French police have arrested nine people after a suspected Islamist sympathiser beheaded a school teacher on the street of a Paris suburb in a shocking act that President Emmanuel Macron said bore the hallmarks of "an Islamist terrorist attack".

Investigators were trying to establish whether the attacker, who was shot dead by police, had acted alone or if he had accomplices, police sources say.

The 18-year-old had approached students in the street and asked them to point out his victim, anti-terrorist prosecutor Jean-Francois Ricard revealed yesterday.

Mr Ricard said the attacker, after beheading history teacher Samuel Paty, 47, posted a photograph of the man's body on Twitter, accompanied by a message saying he had carried out the killing.

Witnesses heard the assailant shout "Allahu Akbar", or "God is greatest", a police source said. His victim had this month shown students cartoons of Prophet Muhammad in a civics class on freedom of expression.

Four relatives of the attacker, including a minor, were detained in the immediate hours after the Friday attack in the middle-class suburb of Conflans-Sainte-Honorine, the sources said.

Five more were later detained, among them two parents of students at the College du Bois d'Aulne, where the teacher was employed.

France will react with the greatest firmness over the attack, Prime Minister Jean Castex said yesterday. "Through one of its defenders, it is the Republic which has been struck in the heart by Islamist terrorism," he wrote on Twitter.

"In solidarity with its teachers, the State will react with the greatest firmness so that the Republic and its citizens live, free! We will never give up. Never."

The previous day, speaking near the scene where the teacher was decapitated, a visibly moved Mr Macron said "the entire nation" stands ready to defend teachers and that "obscurantism will not win".

The attack took place late on Friday afternoon near the middle school where the teacher worked in Conflans-Sainte-Honorine, a north-western suburb around 30km from central Paris.

Police arrived at the scene after receiving a call about a suspicious individual loitering near the school, a police source said.

They discovered the dead teacher and soon spotted the suspect, armed with a blade, who threatened the officers as they tried to arrest him. They opened fire and he later died of his injuries.

Identification documents found on the suspect showed he was an 18-year-old born in Moscow but from Russia's southern region of Chechnya.

The history teacher's recent class discussion on freedom of expression, during which he showed his students cartoons of Prophet Muhammad, had prompted complaints from some parents.

According to a source, the two detained parents had signalled their disagreement with the teacher's decision to show the cartoons.

Muslim leaders have condemned the killing, which many public figures perceived as an attack on the essence of French statehood and its values of secularism, freedom of worship and freedom of expression.

Mr Tareq Oubrou, the imam of a Bordeaux mosque, denied that the killing marked a clash of civilisations.

"It is not a civilisation that kills an innocent person - it is barbarity," he told France Inter, adding that the litany of deadly attacks by Islamist militants or their sympathisers was devastating for France's Muslim community.

"Every day that passes without incident we give thanks," he said. "We are between hammer and anvil. It attacks the Republic, society, peace and the very essence of religion, which is about togetherness."

At the school, parents and teachers paid tribute to the victim.

"According to my son, he was super nice, super friendly, super kind," said Mr Nordine Chaouadi, a parent, adding that his son said the teacher had "simply said to the Muslim children, 'Leave, I don't want it to hurt your feelings.'"

The attack came as a trial is in progress over the January 2015 massacre at the offices of the Charlie Hebdo satirical magazine, which had published caricatures of the Prophet that unleashed a wave of anger across the Islamic world.

The magazine had defiantly re-published the cartoons in the run-up to the trial's opening last month.

In the same month, a young Pakistani wounded two people with a meat cleaver outside the magazine's former offices.

REUTERS, AGENCE FRANCE-PRESSE