The flames and smoke plumed from the spire and roof of the gothic cathedral, visited by millions of people a year.
A spokesman for the cathedral told AFP that the wooden structure supporting the roof was being gutted by the blaze.
The fire service said the blaze could be “potentially linked” to ongoing renovations.
President Emmanuel Macron cancelled a major televised policy speech he was due to give on Monday evening over the “terrible fire ravaging Notre-Dame.”
Paris mayor Anne Hidalgo in a tweet described it as a “terrible fire”.
“The Paris fire service is trying to control the flames,” she wrote on Twitter, asking residents to respect the security cordon thrown up around the site.
A spokesperson for the cathedral said the fire, which comes as French Catholics prepare to celebrate Easter, broke out at around 1650 GMT.
Fire trucks could be seen speeding through Paris towards the scene on the Ile de la Cite — an island in the Seine at the heart of Paris.
US President Donald Trump in a tweet said: “So horrible to watch the massive fire at Notre Dame Cathedral in Paris. Perhaps flying water tankers could be used to put it out. Must act quickly!”