The blaze erupted at around 16:30 GMT on Monday, causing the wood-and-lead spire to collapse onto the roof. Locals have been reported by various news agencies as saying that the entire rooftop has been destroyed in the fire.
Nobody was injured, Junior Interior Minister Laurent Nunez told Reuters at the scene.
"It's too early to determine the causes of the fire," he added, but France 2 television reported that the police were treating it as an accident.
The inferno broke out while the structure was undergoing renovation.
‘Northern tower in danger’
The agency cited a firefighter as saying that efforts were underway to prevent the cathedral’s northern tower from collapsing.
Paris fire brigade chief Jean-Claude Gallet, however, said, "We can consider that the main structure of Notre-Dame has been saved and preserved." It appeared that the two main towers of the building have now been saved, he added.
Later in the day, AFP reported that the fire had destroyed the cathedral’s roof, but firefighters had “won” the battle to save the building’s main structure.
President Emmanuel Macron cancelled an address to rush to the area. "Like all our compatriots, I am sad this evening to see this part of all of us burn," he tweeted.
Macron also vowed that "we will rebuild" Notre-Dame and expressed relief that "the worst had been avoided."
Smoke billowed and ashes flew across a large part of the city following the blaze.
Witnesses said the whole island of the Ile de la Cite in River Seine, which houses the structure, was being evacuated.
Trump’s solution nixed
Donald Trump, meanwhile, suggested that water-bombing aircraft be “quickly” deployed to douse the blaze.
The French Civil Security service, however, spurned the solution, apparently responding to the US president. "Helicopter or plane, the weight of the water and the intensity of dropping it at low altitude could weaken the structure of Notre-Dame and cause collateral damage to surrounding buildings," it tweeted.
UNESCO, which has listed the building as a World Heritage Site, said in a tweet that it was "closely monitoring the situation and is standing by France's side to safeguard and restore” it.
The Vatican said the blaze had caused "shock and sadness."
The cathedral dates back to the 12th century. It is hailed as one of the finest examples of French Gothic architecture.