The blaze collapsed the cathedral's spire and spread to one of its landmark rectangular towers, but Paris fire chief Jean-Claude Gallet said the church's structure had been saved after firefighters managed to stop the fire spreading to the northern belfry.
Margareta, Custodian of the Crown of Romania, who is now undertaking an important visit to France, sent a message to President Emmanuel Macron as soon as the extent of the fire became clear.
Followingthe demise of Napoleon's empire in 1815, France - and Notre Dame - lapsed into neglect and turmoil.
President Donald Trump, 72, suggested using "flying water tankers" to put out the massive fire at Notre Dame Cathedral in a viral tweet on April. 15, and it didn't take long for many of his followers to mock his words with various memes and other responses.
But they were crumbling before the renovation started, and it's unknown whether they will survive the fire.
Queen Elizabeth II said, "Prince Philip and I have been deeply saddened to see the images of the fire which has engulfed Notre-Dame Cathedral".
Gasps and cries of "Oh my god" erupted around an hour after the fire first broke out when the top portion of the church's spire came crashing down.
The cause of the blaze was not immediately confirmed, but the cathedral had been undergoing intense restoration work to help the 850-year-old Gothic masterpiece better deal with the tests of time.
French prosecutors said it was being now being treated as an accident. We express our closeness to the French Catholic and to the people of Paris.
Notre Dame has been a seminal part of Paris' skyline since construction completed in 1260. "It is a heritage site for the culture and religion of France and humanity as a whole".
"There are a lot of art works inside ... it's a real tragedy", Paris Mayor Anne Hidalgo told reporters at the scene. According to the cathedral's official history, revolutionaries tore down 28 statues of kings housed in Notre Dame, driven by anti-monarchist fervor.
French President Macron: 'We'll rebuild Notre-Dame together'
The Notre Dame cathedral is known for its gorgeous stained-glass windows.
The Vatican on Monday expressed its "incredulity" and "sadness" over the fire.
Over the next 150 years, the cathedral would see more facelifts, including the restoration of stained glass in the nave and a decade-long effort to clean the building's western facade.
One firefighter was injured in the blaze, the fire brigade said, revising an earlier assessment that the individual was in a serious condition.
"So disgusting to watch the massive fire at Notre Dame Cathedral in Paris", tweeted US President Donald Trump. "Must act quickly!" Donald's tweet read.
But France's civil security service, which oversees crisis management in the country, tweeted back at Trump that the use of water-bombing aircraft was not being considered.
"If used, (this) could lead to the collapse of the entire structure of the cathedral", it said.
During the French Revolution in the 18th century, the cathedral was vandalised in widespread anti-clerical violence: its spire was dismantled, its treasures plundered and its large statues at the grand entrance doors destroyed.
She walked by the cathedral Monday morning, on her way to Shakespeare and Company, the famous English-language bookstore located within eyeshot of Notre-Dame. Some two thirds of the roof have caved in and the spire toppled over, causing further damage to Notre Dame's interior. Art experts around the world reacted with horror to news of the fire that ravaged cathedral on Monday, April 15, 2019.
"I've visited it so many times, but it will never be the same".