A city hall spokesman said that the area was being cleared, while the fire department said a major operation was under way.
"I see no hope for the building", said witness Jacek Poltorak, watching the fire from a fifth-floor balcony two blocks from the southern facade of the cathedral, one of France's most visited places.
Notre Dame is a historic cathedral known for its French Gothic architecture.
Paris Mayor Anne Hidalgo sent out a message to residents of the city via Twitter: "A bad fire is underway at Notre-Dame Cathedral in Paris". The cathedral is located on the corner of Ile de la Cite, on a spit of land that juts out into the river Seine.
The BBC reported that renovation work is underway at the cathedral, which was built between 1160 and 1260.
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The cause of the fire had not yet been determined Monday afternoon.
French President Emmanuel Macron canceled an address to the nation. Scaffolding on the facade of the building was engulfed in flames as it and the roof of the church started to collapse. The 750-ton spire, a 19th century reproduction of the original Frstructure, collapsed through the roof of the church.
Macron was also headed to the scene after postponing a much-anticipated speech on planned policy measures following a long series of violent protests by the Yellow Vest movement that rocked the capital.
Construction on Notre-Dame Cathedral began in 1163 and concluded in 1345.
The mayor says city officials are in touch with Roman Catholic diocese in Paris.
This story is breaking and will be updated.