The Paris fire service is not sure it will be able to halt the spread of the fire consuming the roof of Notre-Dame cathedral, a senior fire official at the scene told journalists on Monday night.
"It's not certain we'll be able to halt the spread toward the northern bell tower. If that collapses, you can imagine the extent of the damage," General Jean-Claude Gallet said, adding that 400 firefighters were battling the blaze.
Deputy Interior Minister Laurent Nunez, also speaking at the scene, added that saving the gothic monument "is not certain."
A huge fire swept through the roof of the famed cathedral Monday evening, sending flames and huge clouds of grey smoke billowing into the sky.
The flames and smoke plumed from the spire and roof of the gothic cathedral, visited by millions of people a year.
A spokesman for the cathedral told AFP that the wooden structure supporting the roof was being gutted by the blaze.
The fire service said the blaze could be "potentially linked" to ongoing renovations.
President Emmanuel Macron cancelled a major televised policy speech he was due to give on Monday evening over the "terrible fire ravaging Notre-Dame."
Meanwhile, crowds of stunned Parisians and tourists -- some crying, others offering prayers -- watched on in horror as flames engulfed the cathedral.
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 has 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."
Police were attempting to clear 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 kept trying to approach, snarling traffic as they massed on the stone bridges leading to the islands.
Another woman passed by, tears streaming from behind her glasses, too overwhelmed to speak to reporters.
"It's finished, we'll never be able to see it again," said Jerome Fautrey, a 37-year-old who had come to watch.
"Now we need to know how this happened -- with everything that's going on in the world, why Notre-Dame? Maybe it's a message from on high," he said.
- '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."
Her mother, Mary Huxtable, 73, said: "This (Notre-Dame) was on my bucket list to see. Now I'll never go inside."
A short distance away stood another British family, also from London.
"It's devastating," said Nathalie Cadwallader, 42, who had come to Paris two days earlier with her husband and two children for a weeklong visit.
"This is a really historic skyscape and it's horrible this happened, on top of everything else Paris has gone through recently," she said, referring to the deadly jihadist terror attacks that struck the city in 2015.
Her family had initially planned to visit Notre-Dame on Monday but opted instead for the Eiffel Tower, intending to go inside the cathedral on Tuesday.