Two people have died and at least one other person has been wounded in a stabbing in Nice, French reports say.
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.