Smouldering ash surrounds the stone altar in the transept of the cathedral – which dates back to 1163 – as flames illuminate the building’s stone walls.
Hundreds of firefighters battled to gain control of the fire after it began in the early evening and raged until around 3am (2am GMT).
“The worst has been avoided,” French President Emmanuel Macron told reporters at the scene shortly before midnight.
Macron said France would launch a campaign to rebuild the cathedral, which is considered to be among the finest examples of French Gothic cathedral architecture.
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In a message to Macron on Tuesday morning, the Queen said she was “deeply saddened to see the images of the fire which has engulfed Notre-Dame Cathedral” and extended her “sincere admiration to the emergency services”.
A French billionaire, Francois-Henri Pinault, announced a pledge of €100 million (£86.3m) to help efforts to rebuild Notre Dame.
A second French billionaire, Bernard Arnault, and his business LVMH, have pledged €200m (£173m) towards Notre Dame’s reconstruction.
“We will rebuild it together. It will undoubtedly be part of French destiny and our project for the years to come,” a visibly moved Macron added.
Broadcaster Andrew Neil said the destruction of Notre Dame’s wood-timber roof was “the greatest loss”.
“Medieval craftsmen built it using 5,000 oak trees. It lasted til tonight,” he said.
The cathedral’s main stone structure had escaped complete destruction by the time the fire came under control.
“We will continue to watch over any residual pockets of fire and cool down the areas that are still red-hot, like the wooden beam framework,” a fire service spokesman said in the early hours of Tuesday.
Distraught Parisians and stunned tourists gazed in disbelief as the inferno raged at the cathedral, which sits on the Ile de la Cite, an island in the River Seine and marks the very centre of Paris.
Thousands of onlookers lined bridges over the Seine and along its embankments, held at a distance by a police cordon. Some sang liturgical music in harmonies late into the night as they stood vigil, while others recited prayers.
World leaders expressed shock and sent condolences to the French people.
A huge plume of smoke wafted across the city and ash fell over a large area. People watching gasped as the spire folded over onto itself and fell into the inferno.
Firefighters battled smoke and falling drops of molten lead as they tried to rescue some of Notre-Dame’s treasures.
A centuries-old crown of thorns made from reeds and gold and the tunic worn by Saint Louis, a 13th century king of France, were saved, Notre-Dame’s top administrative cleric, Monsignor Patrick Chauvet, said. But firefighters struggled to take down some of the large paintings in time, he said.
The Paris prosecutor’s office said it had launched an inquiry into the fire. Several police sources said they were working on the assumption for now that the fire was accidental.
Macron cancelled an address to the nation that he had been due to give on Monday evening in a bid to answer a wave of street protests that has rocked his presidency. Instead he went to the scene of the blaze with his wife, Brigitte, and some of his ministers. He thanked and congratulated firefighters.
The French Civil Security service, possibly responding to U.S. 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.
German Chancellor Angela Merkel called the cathedral a “symbol of France and our European culture.” British Prime Minister Theresa May said her thoughts were with the French people and emergency services fighting the “terrible blaze.”
The Vatican said the fire at the “symbol of Christianity in France and in the world” had caused shock and sadness and said it was praying for the firefighters.