The cathedral is 850 years old, and last year the Catholic Church launched an urgent appeal for funds to fix the building, which was showing serious signs of decay. One of its famed spires heartbreakingly burned down at 8:07pm local time, framed by a plume of brown smoke that shot up high into the air. "Like all our compatriots, I am sad this evening to see this part of us burning".
French President Emmanuel Macron canceled an address to the nation.
"There will be nothing left", he said. Paris we are with you and will be there to see it restored. It was unclear Monday afternoon if anyone has been injured in the blaze.
French historian Camille Pascal told BFM broadcast channel the blaze marked "the destruction of invaluable heritage".
"For 800 years the Cathedral has watched over Paris", he said.
Notre Dame is a functioning Catholic church and the site of many important religious and ceremonial events.
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"We can be only horrified by what we see". "Heartbroken for Notre-Dame and Paris".
He recalled that Notre Dame bells sounded the death knell following the 2015 Paris attacks.
It has given its name to one of the country's literary masterpieces.
It is not yet clear how serious the blaze is or what caused the fire.
In the cathedral, there are statues: At the Mausoleum of Claude-Henry d'Harcourt, the lieutenant-general of the king's armies who died at age 65 is captured eternally in a white marble sculpture from 1776. It is one of the great treasures of the world.
US President Donald Trump has suggested "perhaps flying water tankers" to extinguish the fire.
British Prime Minister Theresa May expressed her thoughts for the people of France and emergency services battling the devastating fire. "Built in the 12th and 13th centuries, the cathedral is now undergoing extensive renovation work". But the Archdiocese of Paris said it can't afford all the repairs, which was estimated at $185 million.