Notre Dame attack video: Moment Paris police rush to Cathedral as 2,000 trapped inside
According to local media, police have shot a man who attempted to attack an officer with a hammer outside the cathedral.
He is now in hospital, police confirmed.
About a couple of thousand of people are inside, crouching down between the pews, a British witness said.
Police told everybody in the cathedral to raise their arms to show they were not armed.
Paris police confirmed that one of their officers has been injured in the incident.