Flames ravaging the roof illuminated the outline of the monument's two square towers in a fiery glow, and were reflected in the waters of the Seine.
Along the Pont au Change bridge, which connects the Ile de la Cite with the Right Bank, the atmosphere was one of a vigil as hundreds of people watched in hushed silence as smoke rose into the night sky.
Many were quietly singing an Ave Maria in Latin, including Stephane Seigneurie, 52, who said he has lived in Paris for the past 25 years.
"I come often, and go in even where there's no mass because it's an extraordinary place, entwined in the history of France," he said.
"Politically, intellectually and spiritually, it's a symbol of France."
When Seigneurie says that he's very sad, an elegant woman with dark bobbed hair who is crying whispers to him, "We have to pray."
Jeanne Duffy, 62, had travelled from New York to Paris with her twin daughters to see her nephew run the Paris marathon on Sunday.
The girls had wanted to climb the church's towers Monday evening but at the last minute the three decided to go to Disneyland Paris instead.
"We were heartbroken because as New Yorkers we've been through this," Duffy said, referring to the September 11, 2001 attacks which destroyed the twin towers of the World Trade Center.
"In terms of heritage this is much worse. This is a world treasure. Everyone knows Notre-Dame," she said.
Gasps and cries of "Oh my god" erupted at 7:50 pm (1750 GMT) when the top portion of the church's spire came crashing down into an inferno that had spread to the entire roof.
More gasps came a few seconds later when the rest of the spire collapsed, caught on the cameras of thousands of mobile phones.
"Paris is disfigured. The city will never be like it was before," said Philippe, a communications worker in his mid-30s, who had biked over after being alerted of the fire by a friend.
"I'm a Parisian, my father was a Parisian, my grandfather as well -- this was something we brought our sons to see," he said. "I won't be showing this to my son."
"It's a tragedy," he added. "If you pray, now is the time to pray."
Throngs of onlookers remained behind police cordons on the stone bridges leading to the islands
Police cleared pedestrians away from the two islands in the river Seine, including the Ile de la Cite which houses the soaring Gothic church, one of Europe's best-known landmarks.
But throngs of onlookers remained behind police cordons on the stone bridges leading to the islands and along the banks of the Seine river as darkness fell.
"It's finished, we'll never be able to see it again," said Jerome Fautrey, a 37-year-old who had come to watch.
'History up in smoke'
"It's incredible, our history is going up in smoke," said Benoit, 42, who arrived on the scene by bike.
Sam Ogden, 50, had arrived from London on Monday with her husband, their two teenaged sons, and her mother. They had come to Paris specifically to see Notre-Dame, part of a world tour over years to see historic sites.
"This is really sad -- the saddest thing I've ever stood and watched in my life," Ogden said.
She said the fire looked tiny at the beginning, "then within an hour it all came down."
Emilia Freitas, a French teacher from Portugal, was visiting Paris with her husband and daughter, who is studying architecture.
"We were very sad because it's a very important monument, and also worried because many things have happened in Paris lately," she added, referring in particular to the jihadist terror attacks that have struck the city since 2015.
Miguel-Angel Godia, his wife Esther Fajardo and their daughter Raquel, 10 -- who had seen the animated "Hunchback of Notre-Dame" film -- had planned on visiting the church for the first time Tuesday.
"It's a real shame... it's something so immense, so emblematic," said Esther, wiping tears from behind her glasses with the tip of a scarf.