Authorities said they suspected the fire was caused by accident. President Emmanuel Macron declared that the cathedral would be rebuilt. Industrialists pledged hundreds of millions of euros. Ordinary people sang, wept and prayed below the cathedral that has towered above the capital for more than 800 years.

More than 400 firemen were needed to tame the inferno that consumed the roof and collapsed the spire of the gothic masterpiece. They worked through the night, finally quelling the blaze early on Tuesday, some 14 hours after it began.

"Yesterday we thought the whole cathedral would collapse. Yet this morning she is still standing, valiant, despite everything," said Sister Marie Aimee, a nun who had hurried to a nearby church to pray as the flames spread.

"It is a sign of hope."

From the outside, the imposing bell towers and outer walls, with their vast buttresses, stood firm, though the insides and the upper structure had been eviscerated.

Paris public prosecutor Remy Heitz said there was no obvious indication the fire was arson. Fifty people were working on what would be a long and complex investigation, officials said.

One firefighter was injured but no one else was hurt, with the fire starting about 6.30pm local time on Monday after the building was closed to the public for the evening.

The fire swiftly ripped through the cathedral's oak roof supports, where workmen had been carrying out extensive renovations to the spire's timber-framed supports.

The Paris prosecutor has opened an investigation into "involuntary destruction by fire". Police on Tuesday began questioning the workers involved in the restoration, the prosecutor's office said.

Hundreds of stunned onlookers had lined the banks of the river Seine late into Monday night, reciting prayers and singing as they stood in vigil while the fire raged.

Messages of condolence flooded in from around the world.

Pope Francis, the leader of the Catholic Church, was praying for those affected, the Vatican said.

Britain's Queen Elizabeth expressed deep sadness while Prince Charles said he was "utterly heartbroken". Chancellor Angela Merkel offered German help to rebuild a part of "our common European heritage".

Considered among the finest examples of European Gothic architecture, Notre-Dame is visited by more than 13 million people a year. It sits on an island in the Seine, overlooking the Left Bank hangouts of Ernest Hemingway and Pablo Picasso.

The cathedral is owned by the state and has been at the centre of a years-long row between the nation and the Paris archdiocese over who should finance restoration work to collapsed balustrades, crumbling gargoyles and cracked facades.

Within 24 hours, companies and local authorities had pledged more than 700 million euros ($A1 billion) to rebuild it, including 500 million from the three billionaire families that own France's giant luxury goods empires: Kering, LVMH and L'Oreal.

It was too early to estimate the cost of the damage, said the heritage charity Fondation du Patrimoine.

Officials breathed a sigh of relief that many relics and artworks had been saved. At one point, firefighters, policemen and municipal workers formed a human chain to remove the treasures, including a centuries-old crown of thorns made from reeds and gold, and the tunic believed to have been worn by Saint Louis, a 13th century king of France.

Australian Associated Press