Photos and videos of the fire taken by onlookers and circulating on social media offer glimpses of the smoke and flames.
The cause of the blaze isn't yet known, but scaffolding could be seen on the roof of the burning structure.
Construction to build the cathedral got underway in the 12th century, though the current arrangements of are mostly the result of renovations carried out in the 19th century following damage during the French Revolution. Thinking of all Catholics and all French people.
Pedestrians are flooding the streets to watch the blaze, many in tears, one Paris resident tells PEOPLE.
The cathedral is 850 years old, and last year the Catholic Church launched an urgent appeal for funds to fix the building, which was showing serious signs of decay. Firefighters eventually were able to assemble fire boats on the river.More news: Leafs' Kadri could face suspension for high hit
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Notre Dame was constructed in 1163 during the reign of King Louis VII and was completed in 1345. We are mobilized on site in close connection with the Diocese of Paris.
Macron tweeted "emotion of a whole nation" on seeing Notre-Dame ablaze. "Like my countrymen, I am sad watching this part of us burn this evening", he said on Twitter.
He said: "Perhaps flying water tankers could be used to put it out", adding: "Must act quickly!"
It's also famous in literature - the centrepoint of Victor Hugo's novel, The Hunchback of Notre Dame.
Hidalgo urged residents of the French capital to stay away from the security perimeter around the Gothic-style church. It has been modified on numerous occasions throughout the centuries since.
This story is developing and will be updated.