Two people have died and at least one other person has been wounded in a stabbing in Nice as the country prepares to enter into a second national lockdown.
From Friday people in France will only be allowed to leave home for essential work or medical reasons.
President Emmanuel Macron said the country risked being “overwhelmed by a second wave that no doubt will be harder than the first”.
Nice Mayor Christian Estrosi said that one person had been arrested.
He said everything pointed to a “terrorist attack at the heart of the Notre-Dame basilica”.
The interior minister appealed to people to avoid the area in the centre of the French Riviera city. Gérald Darmanin said he was convening a crisis meeting at the ministry in Paris.
According to French reports, a woman and a man had died in the attack and the suspect was detained 10 minutes later before being taken to hospital.
A minute’s silence was held in the National Assembly, where Prime Minister Jean Castex had just been giving details of lockdown measures coming into force on Thursday night.
“Without question this is a very serious new challenge that is striking our country,” he said, appealing for unity and cohesion.