The medieval roof of the cathedral was almost entirely destroyed but photographs from inside show that much of the structure remained intact, after an hours-long firefighting mission.
One firefighter was reported to have been seriously injured in the blaze. The sacred objects and artwork stored in the cathedral were secured.
Emmanuel Macron has committed to rebuild the cathedral, and has promised an international fundraising effort to restore it to its previous state.
While the cause of the fire is still unclear, authorities said they were believed it to have been started by accident.
Initial reports suggest the fire was started in the cathedral's attic, and the visible parts of the blaze do appear to be coming from the centre of the roof. That area has scaffolding around it for building work, as can be seen on this photo.
Numerous reports in French media suggest authorities are concerned that the blaze is connected to the renovation work, which is the reason for that scaffolding around the spire.
Paris Mayor Anne Hidalgo says firefighters are trying to contain a "terrible fire" at the city's Notre Cathedral.
An AP reporter at the scene of Monday's fire says the roof at the back of the cathedral, behind the nave, is in flames and yellow-brown smoke and ash fill the sky.
Hidalgo urged residents of the French capital to stay away from the security perimeter around the Gothic-style church. The mayor says city officials are in touch with Roman Catholic diocese in Paris.
The Île de la Cité – the natural island on which the cathedral stands – has been fully evacuated. People are gathering in nearby squares to watch on as the smoke billows across the entire city.
Macron's cancelled speech was pre-recorded, and was set to be aired later Monday on French TV. He had been expected to lay out his plan to address the citizen complaints that gave rise to the yellow vest protests that have rocked France since November.
Various commentators on Twitter – particularly alt-right voices from the US – appear to be trying to push the suggestion that the fire might have been started intentionally. There is no suggestion at all that it was, and emergency services are still treating the fire as an accident.
