Hundreds of firemen battled the blaze - which consumed the roof and collapsed the eight-centuries-old cathedral's spire - for more than eight hours before bringing it under control, saving its bell towers and outer walls.

A spokesman for Paris firefighters said that "the entire fire is out".

The Paris prosecutor's office said it had launched an inquiry. Several police sources said they were working on the assumption for now that the fire was accidental.

Firefighters who entered the burning cathedral saved many of its treasures, although some paintings remained inside and risked smoke and water damage.

The fire tore through the cathedral's timbered roofing, where workmen were carrying out extensive renovations to collapsed balustrades and crumbling gargoyles and the spire's wooden frame.

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

President Emmanuel Macron promised France would rebuild Notre-Dame, considered among the finest examples of French Gothic cathedral architecture and visited by more than 13 million people annually.

"We will rebuild it together. It will undoubtedly be part of French destiny and our project for the years to come," Mr Macron said outside the cathedral.

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, were saved, Notre-Dame's top administrative cleric, Monsignor Patrick Chauvet, said.

Copper statues representing the Twelve Apostles and four evangelists were removed by crane last week as part of the renovation work.

American tourist Susan Hargrove said she'd been left breathless by the scale of devastation.

"We are talking of world history, of our Western culture but also of something that is truly universal," she said.

"Notre-Dame means something to everybody."

Meanwhile, the fire that devastated Notre-Dame Cathedral in Paris has prompted fundraising appeals in the United States, as people horrified by the blaze began making commitments to restore a global landmark even before the flames are extinguished.

The New York-based French Heritage Society and the Go Fund Me crowdsourcing platform were among the first to offer help for a cathedral that is a must-see destination for visitors to Paris.

French President Emmanuel Macron said an international campaign would be launched to raise funds for the rebuilding of Notre-Dame Cathedral.

The French Heritage Society, an American non-profit group dedicated to preserving French architectural and cultural treasures, launched a web page to raise money for the cathedral's restoration.

"Notre-Dame is obviously an architectural marvel and most certainly a monument that should be restored," Jennifer Herlein, the executive director of the society, said.

Herlein could not immediately say how much her organisation had raised for Notre-Dame. Eventually, the funds raised will go directly to the cathedral, she said.

At the website GoFundMe, more than 50 campaigns related to the cathedral fire had been launched globally on Monday, John Coventry, a spokesman for Go Fund Me, said.

Some of the Go Fund Me campaigns had not listed any money raised by late Monday, and several joke campaigns were created through Go Fund Me to help Quasimodo, the fictional character in Victor Hugo's 19th century novel The Hunchback of Notre-Dame.

Notre-Dame Cathedral has looked to international donors for past renovation efforts.

In 2017, Michel Picaud, president of Friends of Notre-Dame De Paris, told the New York Times his group planned to organise gala dinners, concerts and other events to raise funds in France and the US for restoration work at the cathedral.

Meanwhile, the family investment firm of French luxury goods magnate Francois- Henri Pinault will contribute 100 million euros ($A158 million) towards the rebuilding of the cathedral, news agency AFP has reported.

"Faced with such a tragedy, everyone wishes to give life back as soon as possible to this jewel of our heritage," Pinault wrote in a press statement.

Pinault is president and chief executive of the luxury goods firm Kering, which owns brands including Gucci, Saint Laurent and Balenciaga. The contribution will come from the Pinault family's investment firm.

Earlier: French authorities say firefighters may not be able to save Notre-Dame Cathedral.

The fast-moving fire that consumed the cathedral on Monday has devastated the Parisian landmark which sits in the middle of the city.

Flames that began in the early evening burst rapidly through the roof of the centuries-old cathedral and engulfed the spire, which collapsed, quickly followed by the entire roof.

A firefighter at the scene said all efforts were now being directed at saving the artwork at back of the cathedral and preventing the collapse of the northern tower.

A huge plume of smoke wafted across the city and ash fell over a large area. Parisians watching from the other side of the River Seine gasped as the spire folded over on to itself and fell into the inferno.

At around 7.30pm local time (0430 AEST), nearly three hours after the fire started, a Fire Department spokesman said the next 90 minutes would be crucial in seeing if the blaze could be contained.

Firefighters tried to contain the blaze with water hoses and cleared the area around Notre-Dame, which sits on an island in the River Seine and marks the very centre of Paris. Witnesses said the whole island, the Ile de la Cite, was being evacuated.

"Everything is collapsing," a police officer near the scene said as the cathedral continued to burn.

French President Emmanuel Macron, who cancelled an address to the nation that he had been due to give on Monday evening, went to the scene of the blaze and talked to officials trying to contain it.

The French Civil Security service, possibly responding to US President Donald Trump's suggestion that firefighters "act quickly" and employ flying water tankers, said that was not an option as it might destroy the entire building.

"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.

Foreign leaders sent messages of support.

German Chancellor Angela Merkel and British Prime Minister Theresa May said their thoughts were with the French people. Merkel called the cathedral a "symbol of France and our European culture".

The Vatican said the blaze had caused "shock and sadness" and said it was praying for the firefighters.

The cathedral, which dates back to the 12th century, features in Victor Hugo's classic novel The Hunchback of Notre-Dame. It is a UNESCO World Heritage site that attracts millions of tourists every year.

It is a focal point for French Roman Catholics who like Christians around the world are celebrating Holy Week, marking the death and resurrection of Jesus.

The archbishop of Paris called on all priests in Paris to ring church bells as a gesture of solidarity for Notre-Dame.

The cathedral was in the midst of renovations, with some sections under scaffolding, and bronze statues were removed last week for works.

Built over a century starting in 1163, Notre-Dame is considered to be among the finest examples of French Gothic cathedral architecture.

It is renowned for its rib vaulting, flying buttresses and stunning stained glass windows, as well as its many carved stone gargoyles.

Its 100-metre-long roof, of which a large section was consumed in the first hour of the blaze, was one of the oldest such structures in Paris, according to the cathedral's website.

"There are a lot of art works inside ... it's a real tragedy," Paris Mayor Anne Hidalgo told reporters at the scene.

Australian Associated Press