Police in Paris are responding to an alert at the Notre Dame Cathedral amid reports of gunshots and panic.
Police have closed the area and are asking people to stay away. Eyewitnesses reported tourists fleeing for cover.
Officials have told people to stay away from the area, one of the French capital's most popular tourist destinations.
Reports suggest a man attacked a police officer with a hammer and was later shot. BFM TV reported that a policeman shot a man who tried to assault him. Reports say the attacker has been injured and his condition is not clear.
France is in a state of emergency since attacks by Islamists in Paris left 130 people dead in 2015.
A witness at the Notre Dame incident, David Rahul Métreaum, told BFM TV that he could see a 'man on the ground who is not moving'.
He added: 'There is a group of people from the emergency services around the man on the ground.'
Large numbers of police cars filled the area on the Île de la Cité island in the river Seine.
Matthew Currie Holmes, who was in London during the London Bridge terror attack, tweeted saying: 'So we are trapped in Notre Dame Cathedral. Something is happening outside we don't know what it is. Police sirens can be heard.
'We were just told that the doors will remain locked until the police tell us otherwise. We have no idea what's happening.'
He added: 'Needless to say my family and I are a little on edge. We were in London when the London bridge attacks happened.'
Mark Johnson from Austin, Texas tweeted saying: 'Loud bangs just now at Notre Dame. Crowd scattered. Sirens going off.'
