Flames are shooting out of the roof behind the nave of the cathedral, among the most visited landmarks in the world.
Sights of the flames stopped passers-by in their tracks along the Seine River that passes beneath the cathedral.
French President Emmanuel Macron postponed a televised speech to the nation because of the stunning blaze. Macron's pre-recorded speech was set to be aired Monday evening, to lay out his long-awaited answers to the yellow vest crisis that has rocked the country since last November.
Associated Press reporters at the scene saw massive plumes of yellow brown smoke filling the air above the Cathedral and ash falling on the island that houses Notre Dame and marks the center of Paris.
Paris Mayor Anne Hidalgo is in despair at the "terrible fire." Hidalgo said in a Twitter message that Paris firefighters are still trying to limit the fire and urged Paris citizens to respect the security perimeter that has been set around the cathedral.
Hidalgo said Paris authorities are in touch with Paris diocese.