The fire, which began in the early evening, sent flames and huge clouds of grey smoke billowing into the sky as stunned Parisians and tourists watched in horror.
"Everything is collapsing", a police officer near the scene said as the cathedral continued to burn.
French President Emmanuel Macron postponed a televised speech to the nation because of the stunning blaze.
The idea that firefighters "must act quickly" needed to be said was absurd and offensive when it obviously goes without saying that officials in Paris were working to halt the damage as quickly as possible.
A spokesman for the cathedral said the whole structure was "burning".
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Notre Dame spokesperson Andre Finot told French media outlets that the entire wood frame of the building would likely be lost and the structural integrity would be compromised, according to the AP.
'I have a lot of friends who live overseas and every time they come I tell them to go to Notre Dame, ' said witness Samantha Silva, with tears in her eyes.
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It's unclear if anyone has been hurt in the fire.
"So awful to watch the massive fire at Notre Dame Cathedral in Paris".
US President Donald Trump has suggested "perhaps flying water tankers" to extinguish the fire. "It is one of Paris' most popular attractions, drawing an estimated 13 million visitors a year".
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'There are a lot of art works inside.it's a real tragedy, ' Paris Mayor Anne Hidalgo said at the scene.
Germany's Chancellor Angela Merkel has offered her support to the people of France, calling Notre-Dame a "symbol of French and European culture".
The cathedral is "more than a historical monument", according to the translated cathedral website.
No other site represents France quite like Notre-Dame.
■AFP reported that a part of the Ile de La Cite neighborhood where the cathedral is situated, at the heart of Paris, was being evacuated at around 7:30 p.m. local time.
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