Historians expressed incredulity at the collapse of a building that has been a symbol of France for nearly a millennium and withstood war and revolution.
As it burned into the evening, firefighters battled to prevent one of the main bell towers from collapsing.
Prosecutors believe the fire was an accident and may have started amid a renovation project at the cathedral.
Expressions of shock and mourning came in from around the world.
Yvette, a French-American student, told Al Jazeera she rushed to the scene to see the cathedral one last time "while it was still here". We want to keep our journalism open and accessible and be able to keep providing you with news and analysis from the frontlines of Israel, the Middle East and the Jewish World.
"It's a symbol of Catholic France", said Paris resident Gaetane Schlienger, 18, who tried to climb a tree near the vigil.
Nancy Walsh, of the Dunning neighborhood, said her daughter and about 20 students in the global baccalaureate French class at Taft High School had just arrived in France for a weeklong trip.
"The art, the stained glass windows, the pictures - I mean, there's so much art that they don't even talk about", Seaver told News4Jax.
Quebec Premier François Legault tweeted his condolences, saying "Notre-Dame Cathedral in Paris is a gem!" It has been the most visited monument in France, according to the Paris Convention and Visitors Bureau.
"It's very upsetting", Kei told News4Jax.
Khloe Kardashian Admits She's "Having Anxiety" About True's First Birthday
Over the past year there has been a lot of memorable moments for True , her mom Khloe and her dad Tristan Thompson , and luckily there are photographs to prove it.
Firefighters have managed to save the 850-year-old Gothic building's main stone structure, including its two towers, but the spire and roof have collapsed. It "contains relics for the protection of Paris", he said on Twitter.
A hole left by the fallen spire was still burning and sparks rained down from the cathedral's vaulted ceilings more than five hours after the fire broke out. So I solemnly say tonight: "We will rebuild it together".
Twelve million people visit Notre Dame each year. He said firefighters would keep working overnight to cool down the building. "It remains to be seen whether the vault, which protects the cathedral, will be affected or not". The city's mayor, Anne Hidalgo, said the significant collection of art work and holy objects inside the church had been recovered.
One firefighter was seriously injured, Gallet said.
Built between the years 1163 and 1345, Notre Dame is one of the most popular tourist sites in Paris, drawing around 13 million people every year.
Archbishop of Miami Thomas Wenski wants parishioners to pray for the people in Paris following the fire.
The King of Sweden sent a message to the President of France saying, "Her Majesty the Queen and I wish to express our honest sympathy following the fire at the Cathedral of Notre-Dame de Paris and convey our deep feelings of solidarity to the French people". But he said the cathedral will be rebuilt and that an global fundraising campaign will be launched.
A fire broke out inside the 850-year-old Notre-Dame cathedral in Paris during fix works around 7.00 PM on Monday.
He said the fire, which dominated news coverage, was a bad sight to behold.
U.S. President Donald Trump also tweeted, "Perhaps flying water tankers could be used to put it out", adding, "God bless the people of France!"