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'.
Last year, the Catholic Church in France appealed for funds to save the building, which was crumbling.
Loud bangs could be heard as flames burst through the cathedral's roof, also and destroying its stain-glass windows.
All efforts are now being put into saving the cathedral's artwork and preventing the collapse of its northern tower.
The deputy interior minister Laurent Nunes told reporters it was "not certain" that it could be saved.
