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.
About 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