The spire of the landmark in France’s capital collapsed as the flames ripped through the 850-year-old building. One eyewitness described the dramatic moment the central tower collapsed. Shiv Malik told BBC News: “Oh, oh, the central has just caved in into the Cathedral itself.”
“That will further create a huge complication inside the building itself, no doubt.
“But that has just happened now in front of my very eyes.”
The cause of the blaze at the Gothic monument which dates back to the 12th century is not yet known.
The witness said he could feel the heat from the burning building through a window.
Mr Malik added: “I can feel it from where I am through a window.
“It felt like it had very much started in and around the scaffolding area at the centre of the building.”
French President Emmanuel Macron cancelled a planned address to the nation due to the “terrible fire”.
The French President tweeted: “Our Lady of Paris in flames.
Mayor Anne Hidalgo said on Twitter: “A terrible fire is underway at Notre-Dame Cathedral in Paris.”
“Emotion of a whole nation. Thought for all Catholics and for all French.
“Like all our countrymen, I’m sad tonight to see this part of us burn.”
France 2 television reported that police were treating the incident as an accident.
Officials said it could be linked to renovation works being carried out on the cathedral.
US President Donald Trump tweeted: “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!”
Parts of the building were covered by scaffolding while bronze statues were removed last week.
The cathedral is famous for its part in Victor Hugo’s novel the Hunchback of Notre-Dame.
The landmark attracts millions of tourists every year.