One firefighter has been seriously injured in the Notre Dame blaze, officials have said.
Emergency services have been working to contain the blaze for several hours, after the fire began at around 6.30pm.
Anne Hidalgo, Paris’ mayor, said firefighters were optimistic that they could salvage the cathedral’s iconic main towers.
Notre Dame’s spire collapsed around an hour and a half after the fire began.
Huge crowds have gathered in the city to watch firefighters attempt to extinguish the flames.
The Paris prosecutor’s office has opened an investigation into the fire and believe the fire was started accidentally after a preliminary probe.
"Notre Dame is aflame," French leader Emmanuel Macron said.
"Great emotion for the whole nation. Our thoughts go out to all Catholics and for to the French people.
"Like all of my fellow citizens, I am sad to see this part of us burn tonight."
Additional reporting by agencies
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