Awoman has been decapitated during a knife attack near the Notre Dame Basilica church in the French city of Nice, according to French media citing police sources.
Reports say three people are dead and several others are injured.
The assailant was arrested after the Thursday morning attack and taken to a nearby hospital after being injured during his arrest, a police offical said. He was believed to be acting alone, the official said.
One of the victims - a woman - was beheaded, according to a police source.
The attack began around 9am, according to local reports, before police swarmed the area and arrested the attacker.
The French Interior Minister Gérald Darmanin has confirmed a police operation is underway.
The city's mayor, Christian Estrosi, tweeted: "I am on site with the [police] who arrested the perpetrator of the attack.
"I confirm that everything suggests a terrorist attack in the Basilica of Notre-Dame de Nice."
The lower house of parliament suspended a debate on new virus restrictions and held a moment of silence for the victims.
The reason for the attack is unclear.
The attack is just the latest to strike France, after an 18-year-old Pakistani stabbed a wounded two people outside the old offices of satirical magazine Charlie Hebdo.
The man has admitted to police that he was targeting the magazine for publishing cartoons of the Prophet Mohammed.
Earlier this month a French history teacher, Samuel Paty, was beheaded in another attack north of Paris - also over cartoons of the Prophet.
Paty was stabbed by an 18-year-old Chechen after he showed the cartoons to his students during a lesson on free speech.
Parents of pupils at the school had led a campaign against him, before the attack took place. Seven have been arrested.
- additional reporting Daily Mail