Hundreds gathered outside the cathedral, many in tears, as flames tore through the beloved building, causing the roof to collapse. French President Emmanuel Macron canceled a scheduled speech Monday to deal with the ongoing fire.
"Our Lady of Paris is in flames," Macron said on Twitter. "Emotion for the whole nation. A thought for all Catholics and for all French. Like all our countrymen, I'm sad tonight to see this part of us burn."
Nicolas Marang, 47, has lived in Paris for more than 25 years and took video as Notre Dame's spire collapsed amid the raging fire. He told NBC News he was running along the Seine River when he saw the smoke, which looked like a "dark smog," coming from the Notre Dame.
The scene outside the cathedral was "confusing and devastating," according to Marang.
"Something just collapsed inside me," he said. " One of the worst things I've ever seen."
David Kang, 38, said he was on his way home when he passed by the Notre Dame and noticed the flames. People stopped all around, in shock, as they witnessed the landmark's destruction.
"It was an emotional experience. I see it every day on my way to home, it was so sad," Kang said. "I don’t remember something like this ever happening. Notre Dame is the center of Paris."
Audrey Edwards, 71, an American who retired to Paris in 2017, said she saw the smoke from her apartment window.
"I’m not a particularly religious person. Notre Dame is a masterpiece architecturally, and as a real estate broker, I started to tear up because it’s such a magnificent structure, aesthetically speaking," she said.
Edwards lived in Brooklyn and related the experience of seeing the plumes of smoke Monday to the images of the Twin Towers collapsing in lower Manhattan on 9/11.
"These are the things that you think are going to be eternal, but of course, nothing is," she said.
University of Michigan student Madeline Boersma is living in Paris while she studies abroad at Sciences Po. Boersma, 20, said she was headed home from class but circled back to see the fire.
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"Just walking down the street, you see it all the time," Boersma said. "As I was walking up here I thought, 'I was just walking here three weeks ago and it was so normal.' Today I'm here and everyone is heartbroken."
Gabriella Pilet, an Argentinean who has lived in Paris for 20 years, stood near the Seine and sang the Hail Mary prayer in French as the cathedral burned.
"I came here to honor this place," Pilet said. "For me, it’s a symbol of gathering. Everyone, even if you’re not a Christian, comes here to gather."
Pilet said she regrets never going up to the roof of the church as she had always planned.
"All the way at the end that was my favorite thing, a statue of the Virgin Mary," Pilet said. "Several years ago after the terrorist attacks, they stopped you from visiting some places. I was very upset."
President Donald Trump called the Notre Dame one of the "great treasures of the world" during an economic roundtable event in Minnesota.
The cathedral is "beyond countries, that's beyond anything, that’s part of our growing up, it’s a part of our culture, it’s a part of our lives," Trump said.
Cardinal Timothy Dolan, archbishop of New York, tweeted Monday that he prayed for the Paris landmark after hearing of the fire.
"I just went next door to our own beloved Cathedral, Saint Patrick’s, to ask the intercession of Notre Dame, our Lady, for the Cathedral at the heart of Paris, and of civilization, now in flames! God preserve this splendid house of prayer, and protect those battling the blaze," Dolan wrote.
"The Holy See has seen with shock and sadness the news of the terrible fire that has devastated the Cathedral of #NotreDame, symbol of Christianity in France and in the world," the religious epicenter said.
Pastor Miguel Castellanos of the Notre-Dame Basilica in Montreal, Canada, tweeted his sympathies to those suffering and offered prayers to "everyone who will have to work hard to rebuild an international treasure."
At least 400 firefighters were on the scene, desperately trying to douse the flames that were destroying one of France's most treasured sites.
The New York Fire Department released a statement Monday expressing sympathy with the Paris Fire Brigade as they fought to quell the inferno.
"Images of Notre Dame in flames are an awful sight to behold," the agency said. "The thoughts of the FDNY are with the members of @PompiersParis as they bravely battle this terrible fire in one of the world’s most beautiful and historic houses of worship."
Former President Barack Obama sent out a message of hope alongside a photo of him with his daughters, Sasha and Malia, lighting candles at the cathedral.
"It’s in our nature to mourn when we see history lost – but it’s also in our nature to rebuild for tomorrow, as strong as we can," Obama said.
Notre Dame is one of the world’s great treasures, and we’re thinking of the people of France in your time of grief. It’s in our nature to mourn when we see history lost – but it’s also in our nature to rebuild for tomorrow, as strong as we can. pic.twitter.com/SpMEvv1BzB
Former first lady and Secretary of State Hillary Clinton said in a tweet that her heart went out to Paris.
"Notre Dame is a symbol of our ability as human beings to unite for a higher purpose — to build breathtaking spaces for worship that no one person could have built on their own," Clinton said. "I wish France strength and shared purpose as they grieve and rebuild."
On social media, people across the world began recounting their experiences and appreciation of the cathedral's beauty. One Twitter user said that he first visited the church at 7 years old.
"It's one of the most beautiful things I've ever seen, and I'd visit the cathedral each time I was in Paris just to be moved and astonished. This is heartbreaking," he wrote.
Pop singer Camila Cabello wrote in a tweet Monday that she was devastated watching the cathedral burn.
"I’ll never forget walking in the first time in Paris and being in awe of the beauty of it," Cabello said.
Construction of Notre Dame began in 1163 under the reign of King Louis VII, and the first stone was laid in the presence of Pope Alexander III.
The landmark was not considered complete until almost 200 years later, however, with the installation of flying buttresses and a stone fence surrounding the choir and the sanctuary, according to the website for Notre Dame de Paris.
Christopher Brennan reported from Paris, France. Doha Madani reported from New York City.
Christopher Brennan is a freelance contributor for NBC News.
Doha Madani is a breaking news reporter for NBC News.
Ali Gostanian, Jareen Imam and Rima Abdelkader contributed.