The spire and roof of the 12th century building in Paris have been destroyed
Flames broke out on Monday evening, quickly devastating the spire and roof and sending plumes of black smoke into the sky.
One firefighter has been seriously injured and local media say police are treating the blaze as an accident.
French President Emmanuel Macron said he was "so sad tonight to see this part of all of us burn" and declared a national emergency.
Speaking from the scene in Paris, he expressed sympathy with Catholics "around the world" following the "terrible tragedy" but added the worst had been avoided.
He vowed to launch an international fundraising campaign for the cathedral so the world could rebuild Notre-Dame together.
Thoughts with Paris and our friends in France as they wake to see the damage done to Notre Dame. Such a beautiful icon of design infused with so much identity, culture and heritage. #resurgam #notredame pic.twitter.com/Iz5b3IMdN1— Luke Pollard MP (@LukePollard) April 16, 2019
When heritage goes up in smoke it feels like such a loss, it makes your heart ache, as with The Macintosh building destroyed in Glasgow.. both under repair! Both humanity’s treasures. Fire is so destructive. Check your precautions. Thinking of firefighters everywhere #NotreDame pic.twitter.com/7EWH5HZGNM— Sharron Davies MBE (@sharrond62) April 16, 2019
Nous prions pour la cite de Paris, et la communaute de la Cathedrale de Notre Dame. https://t.co/SqUXh3LTvA— Exeter Cathedral (@ExeterCathedral) April 15, 2019
En route to Cornwall for Chrism Mass this evening, I am saddened to learn of this tragedy; prayers for every member of the Church of Paris and for all throughout the world for whom this church is iconic... https://t.co/YsAnukebGf— Terrence Prendergast (@archterentius) April 15, 2019
Deeply upsetting to see the Parisian monument #NotreDame engulfed by flames.— Vince Barry (@VoteVinceBarry) April 15, 2019
After surviving two world wars, occupation and revolution it's an ignoble fate to befall a beautiful, historic site at the heart of Paris.