Firefighters are battling to save the 850-year-old Gothic building, but its spire and roof have collapsed.
The cause is not yet clear, but officials say that it could be linked to renovation work.
A Paris fire official said the main structure had now been "saved and preserved".
The Paris prosecutor's office said it has opened an inquiry into "accidental destruction by fire."
Loud bangs could be heard as flames burst through the cathedral's roof, also destroying its stained-glass windows.