Images emerged showing the upper parts of the gothic cathedral engulfed by flames. The 90-metre-high spire and parts of the roof completely collapsed, and the fire spread to the bell towers. It took nine hours to bring it under control.
The fire started at approximately 7pm local time and brought hundreds of people out into the street of Paris.
A spokesperson for the cathedral told Associated Press that the cathedral's wooden interior was ablaze and likely to be destroyed.
"A terrible fire"
Paris mayor Anne Hidalgo described it as "a terrible fire" on her Twitter page, and urged onlookers to respect the security boundaries in place while the fire brigade tackled the blaze.
— Mohammed (@MazAlJumaily) April 15, 2019
French president Emmanuel Macron cancelled a speech he was due to give so he could attend the scene. He also made a statement on Twitter, describing the situation as "emotion of a whole nation".
"Thought for all Catholics and for all French. Like all our compatriots, I am sad tonight to see this part of us burn," he said.
Renovation works thought to be cause
The cause of the fire is unconfirmed, but officials told BBC News it might linked to renovation.
The historic cathedral had been in bad condition after decades of damage from weather and pollution. Large cracks and fissures had appeared in the structure, and many of its gargoyles were either damaged or destroyed.
Un dispositif exceptionnel est mis en place pour éteindre l'incendie #NotreDame #Paris : 400 pompiers sont mobilisés. Évitez le secteur et suivez les consignes de @prefpolice. #HérosduQuotidien pic.twitter.com/hfKG8yFSvW
— Ministère de l'Intérieur (@Place_Beauvau) April 15, 2019
The restoration project was estimated to cost €150 million. It was being partially funded by the French state, but additional funds were being sourced from private donors.
The building, constructed in the 12th and 13th centuries, is considered one the world's finest examples of gothic architecture, thanks to its pioneering use of rib vaults and flying buttresses, as well as its striking stained-glass rose windows.
Cathedral could be rebuilt
French officials said they were doing all they could to tackle the blaze. However they said that water-dropping aircrafts could not be used as they posed a threat to the building's structural integrity.
This form of action was suggested by US president Donald Trump on Twitter. "Perhaps flying water tankers could be used to put it out," he wrote.
If destroyed, it would be possible to rebuild the cathedral. Thanks to the work of art historian Andrew Tallon, who passed away in late 2018, laser scans have been used to map the entire structure of the building.
Main image is by AFP/Getty Images.