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Notre-Dame Cathedral went up in flames in central Paris on Monday, but firefighters said they saved the shell of the stone structure from collapse.

Thousands of onlookers on the banks of the River Seine were stunned into silence as the blaze engulfed the world heritage site days before Easter.

A French firefighter official said the only casualty from the fire was one fireman who was seriously injured, Reuters reported.

La flèche de Notre-Dame vient de s'effondrer sous les exclamations horrifiées des gens #NotreDame pic.twitter.com/kfLNt7rhsZ — Wladimir Garcin-Berson (@vladogb) April 15, 2019

President Emmanuel Macron postponed his 8pm televised speech to the nation to head to the site of the fire.

"This Notre-Dame cathedral, we will rebuild it," Mr Macron said. "All together. It's part of our French destiny. I commit myself: tomorrow a national subscription will be launched and well beyond our borders."

Earlier in the evening, the French leader said his thoughts were with all Catholics and all French people.

"Like my countrymen, I am sad watching this part of us burn this evening," he said.

Firefighters said they believed the fire began in the attic of the cathedral. They scrambled to rescue valuable pieces of historical art from inside the building.

The cathedral’s spire collapsed in the flames, which spread to one of its rectangular towers, followed by its roof.

La flèche de Notre-Dame vient de s'effondrer sous les exclamations horrifiées des gens #NotreDame pic.twitter.com/kfLNt7rhsZ — Wladimir Garcin-Berson (@vladogb) April 15, 2019

Notre-Dame was being renovated, with some sections under scaffolding, and bronze statues were removed last week for works.

Fire brigade in Paris said the fire could be linked to the renovations and was being treated as an accident, AFP reported.

World leaders reacted quickly to the crisis.

"My thoughts are with the people of France tonight and with the emergency services who are fighting the terrible blaze at Notre-Dame cathedral," British Prime Minister Theresa May said.

My thoughts are with the people of France tonight and with the emergency services who are fighting the terrible blaze at Notre-Dame cathedral. — Theresa May (@theresa_may) April 15, 2019

German Chancellor Angela Merkel said: "It is with deep sadness that I am watching the events in Paris.

"I am sorry to see these terrible images of Our Dame, symbol of France and our European culture, in flames. Our thoughts are with our French friends."

US President Donald Trump said on Twitter: "So horrible to watch the massive fire at Notre-Dame cathedral in Paris. Perhaps flying water tankers could be used to put it out. Must act quickly."

The monument receives more than 13 million visitors a year and is the city's most visited attraction, beating even the Eiffel Tower, the Paris tourism report says.

It dates back to the 12th century and features in Victor Hugo’s classic novel The Hunchback of Notre-Dame.

It took 200 years to complete the cathedral.