A blaze broke out on the afternoon of April 15 at the Notre Dame Cathedral in central Paris where firefighters were still fighting to put the fire under control, Paris Mayor Anne Hidalgo said.
"A terrible fire is underway at Notre-Dame Cathedral in Paris, (and) firefighters are trying to control the flames," Hidalgo tweeted.
"We are mobilized locally in close connection with the Diocese of Paris. I invite everyone to respect the security perimeter," she added.
Spire of the Notre Dame cathedral later collapsed in the huge fire, Paris Deputy Mayor said.
Online footage showed a thick smoke billowing from the top of the cathedral and huge flames between its two bell towers and ravaged the monument's roofing.
According to local media, the fire, which broke out at around 1700 GMT local time, may be linked to renovation works being carried out at the medieval cathedral, one of the capital's tourist magnets.
No casualties have been reported yet.
Following the blaze, President Emmanuel Macron, due to give a speech at 1800 GMT, delayed his TV appearance, his office said without giving further details.