Paris mayor Anne Hidalgo said firefighters were optimistic they could salvage its main towers from the flames.

But the massive fire that consumed the cathedral on Monday had already destroyed the spire and the roof of the historic Paris landmark.

The cause of the blaze was not known, but French media quoted the Paris fire brigade as saying the fire is "potentially linked" to a $US6.8 million ($9.4 million) renovation project on the church's spire and its 250 tonness of lead. Prosecutors opened an investigation as Paris police said there were no reported deaths.

Flames shot out of the roof behind the nave of the cathedral, among the most visited landmarks in the world. Hundreds of people lined up on bridges around the island that houses the cathedral, watching in shock as acrid smoke rose in plumes.

French President Emmanuel Macron was treating the fire as a national emergency, rushing to the scene and straight into meetings at the Paris police headquarters nearby.

Macron cancelled an address to the nation that he had been due to give on Monday evening.

He wrote on Twitter that Notre Dame was in flames. "Thoughts for all Catholics and for all French people," he wrote. "Like all our compatriots, I am sad tonight to see this part of us burn."

Deputy mayor Emmanuel Gregoire said emergency services were trying to salvage the famed art pieces stored in the cathedral.

Built in the 12th and 13th centuries, Notre Dame is the most famous of the Gothic cathedrals of the Middle Ages as well as one of the most beloved structures in the world.

Situated on the Ile de la Cite, an island in the Seine river, the cathedral's architecture is famous for, among other things, its many gargoyles and its iconic flying buttresses.

Among the most celebrated artworks inside are its three stained-glass rose windows, placed high up on the west, north and south faces of the cathedral. Its priceless treasures also include a Catholic relic, the crown of thorns, which is only occasionally displayed, including on Fridays during Lent.

The cathedral was immortalised in Victor Hugo's The Hunchback of Notre Dame, published in 1831, and has long been a subject of fascination in popular culture as well as the traditional art world.

French historian Camille Pascal told BFM broadcast channel that the blaze marked "the destruction of invaluable heritage."

"It's been 800 years that the Cathedral watches over Paris", Pascal said. "Happy and unfortunate events for centuries have been marked by the bells of Notre Dame."

He added: "We can be only horrified by what we see."

Associated Press reporters at the scene saw massive plumes of yellow brown smoke filling the air above the Cathedral and ash falling on the island that houses Notre Dame and marks the centre of Paris. As the spire fell, the sky lit up orange.

Paris Mayor Anne Hidalgo is in despair at the "terrible fire."

Hidalgo said in a Twitter message that Paris firefighters were still trying to limit the fire and urged Paris citizens to respect the security perimeter that has been set around the cathedral.

At 5.30am AEST, nearly three hours after the fire started, a Paris Fire Department spokesman said the next 90 minutes would be crucial to see if the blaze could be contained.

Hidalgo said Paris authorities were in touch with the Paris diocese.

Reactions from around the world came swiftly.

The Vatican issued a statement about the "terrible fire" that has "devastated" Notre Dame Cathedral in Paris.

The Vatican said: "The Holy See has seen with shock and sadness the news of the terrible fire that has devastated the Cathedral of Notre Dame, symbol of Christianity in France and in the world."

The statement says the Vatican is praying for firefighters "and those who are doing everything possible to confront this dramatic situation" on Monday.

It also expressed "our closeness to French Catholics and the population of Paris, and we assure our prayers for firefighters

In Washington, Trump tweeted: "So horrible to watch the massive fire at Notre Dame Cathedral in Paris" and suggested first responders use "flying water tankers" to put it out.

Cardinal Timothy Dolan, the archbishop of New York, said he was praying "to ask the intercession of Notre Dame, our Lady, for the Cathedral at the heart of Paris, and of civilisation, now in flames! God preserve this splendid house of prayer, and protect those battling the blaze."

Prime minister Scott Morrison expressed concern for the fire and predicted it would be rebuilt.

Eyewitnesses watched in shock as the blaze consumed the cathedral.

"Basically the whole rooftop is gone. I see no hope for the building," said Jacek Poltorak, who was watching the fire from a fifth-floor balcony two blocks from the southern facade of the cathedral.

Donia Hammami, a trade expert in Paris, ran to the scene from her gym nearby when she saw the reports. She was in tears in the crowd, watching the cathedral burn.

"For me, this has been an inspiration for so many other churches in Europe from the 14th century onward, in the way it came up with the a way to mirror more light," Hammami said. "It's been here for all those ages. This is a historic moment for all of us in the worst possible sense of the term."

Australia's ambassador to France, Brendan Berne, tweeted: "Our hearts go out to our French friends in the face of this terrible tragedy. Notre Dame belongs to all who love France - we are with you."

It was not immediately clear what had caused the fire. France 2 television reported that police were treating the incident as an accident. No injuries had been reported.

The fire alarm had gone off around 6.30pm local time, according to André Finot, a spokesman for the cathedral.

Firefighters cleared the area around the cathedral, which sits on an island in the River Seine and marks the very centre of Paris. Nearby buildings were evacuated as firefighters tried to contain the blaze from the ground as well as from cranes.

The Gothic cathedral is famed for its many carved stone gargoyles, stunning stained glass windows and the flying buttresses that hold up its wall.

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

The deputy mayor, Emmanuel Gregoire, said first responders were trying to salvage art and other priceless pieces stored in the cathedral.

British Prime Minister Theresa May said on Monday her thoughts were with France as fire devastated the centuries-old Notre-Dame Cathedral in Paris.

"My thoughts are with the people of France tonight and with the emergency services who are fighting the terrible blaze at Notre-Dame Cathedral," May, who is currently on holiday, said on Twitter.

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.

German Chancellor Angela Merkel said she was deeply saddened by the events in Paris. "Our thoughts are with our French friends."

Paris prosecutors had opened an investigation into fire before the flames had even gone out.

A centre of Roman Catholic faith, over the centuries Notre-Dame has also been a target of political upheaval.

It was ransacked by rioting Protestant Huguenots in the 16th century, pillaged again during the French Revolution of the 1790s and left in a state of semi-neglect. Hugo’s 1831 work led to revived interest in the cathedral and a major – partly botched – restoration that began in 1844.

The wood-and-lead spire was built during that restoration, according to the cathedral's website.

UNESCO said in a tweet it was "closely monitoring the situation and is standing by France's side to safeguard and restore this invaluable heritage".

– AAP, with agencies