Devastating video shows the roof of the building collapsing in the flames.
Around 400 firefighters battled into the night to control the flames, declaring in the early hours of Tuesday that it was partially extinguished but completely under control, around nine hours after it broke out.
As it stands, the fire has already all but destroyed the roof, and the spire has collapsed into the building.
French writer and historian Camille Pascal said, "We can be only horrified by what we see".
Others described the tragedy of never being able to see the Notre Dame as it was before the blaze.
"I just remember seeing it.and realizing how long it had been there and how much it withstood", Volk said.
The cause of the blaze was not immediately clear, but the cathedral had been undergoing intense restoration work which the fire service said could be linked to the blaze.
French prosecutors said it was being treated as an "involuntary" fire, indicating that foul play was ruled out for now. "And we're thinking of the people of France in your time of grief. It's the entire culture, entire history of France incarnated in this monument", Bernard Lecomte, a writer and specialist in religious history told BFM TV.
Chicagoan Erin Rydberg is in Paris for work. Officials later said teams had managed to salvage an unknown quantity of the cultural treasures.More news: Steph Curry breaks National Basketball Association all-time 3-point playoff record in Warriors win
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Macron cancelled an address to the nation that he had been due to give later on Monday evening.
"Our thoughts go out to all Catholics and to the French people".
A center of Roman Catholic faith, over the centuries Notre-Dame has also been a target of political upheaval.
The Vatican on Monday expressed its "incredulity" and "sadness" over the fire.
One firefighter has been seriously injured while working to contain the blaze. Victor Hugo used its gargoyles and flying buttresses as the setting for his 1831 novel The Hunchback of Notre Dame.
"When the spire fell, the crowd reacted with "ohhh" and 'ahh, ' but I guess a lot of them were just shocked silently", Binétruy said.
Following Trump's tweet, the French civil protection service responded that the one thing it was not contemplating was using aerial tankers, which dump enormously heavy loads of water, usually on wildfires.
"All means are being used, except for water-bombing aircraft which, if used, could lead to the collapse of the entire structure of the cathedral".
Flames shot out of the roof behind the nave of the cathedral, among the most visited landmarks in the world. Here's a look at the history of the now fire-damaged edifice that has symbolized the French capital for more than eight centuries. All six were moved to the Louvre in the 1860s, and only "The Visitation" was returned to Notre Dame.
The building survived the devastation of two global conflicts in the 20th century and famously rang its bells on August 24, 1944, the day of the Liberation of Paris from German occupation at the end of the World War II. And while the cathedral - one of the best examples of French Gothic architecture - has been damaged, restored and upgraded many times through the centuries, it has not been on a scale that will be required now.