Video report by ITV News Europe Editor James Mates
The spire of Notre-Dame Cathedral in Paris along with much of its roof has collapsed amid a raging fire that has devastated one of France's most iconic landmarks.
But despite the huge blaze, authorities believe the structure of the 850-year-old Gothic cathedral can be saved.
The fire had spread to one of the cathedral's famous rectangular bell towers, but firefighters said they had prevented it from reaching the second belfry.
Paris fire commander Jean-Claude Gallet said crews will keep working overnight to cool down the structure.
Speaking from the scene on Monday night, French President Emmanuel Macron pledged to rebuild Notre Dame and would be seeking international help to do so.
Emergency services rescued priceless artwork stored in the cathedral, although some had been moved earlier this week as part of restoration work.
The deputy mayor of Paris, Emmanuel Gregoire, says Notre-Dame Cathedral has suffered "colossal damages".
Mr Macron, who cancelled a scheduled televised address to the nation to travel to the scene, expressed his "sadness" at the devastation but said the "worst had been avoided".
The president praised the "courage" and "great professionalism" of firefighters with spared Notre Dame's facade and its belfries from being destroyed.
The French leader said a fundraising campaign to restore Notre-Dame would be launched on Tuesday and called on the world's "greatest talents" to help.
Earlier, the president said the fire was taking part of all of France's people with it.
Mr Macron wrote on Twitter: "Our Lady of Paris in flames. 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."
Paris prosecutors opened an investigation into the blaze which began just before 8pm local time (7pm BST) on Monday and could be "potentially linked" to the $6.8 million renovation of the spire.
They believe the fire began accidentally, based on their preliminary investigation.
Authorities said no-one had been hurt.
Hundreds of Parisians and tourists lined the streets and bridges, watching in shock as the mediaeval building was consumed by flames.
A group, some kneeling, sang hymns while others openly wept.
Religious and world leaders have shared their sorrow at the sight of one of the world's most visited attractions and the centre of French Catholicism up in flames.
In a statement, the Vatican said: "The Holy See has seen with shock and sadness the news of the terrible fire that has devastated the Cathedral of Notre-Dame, symbol of Christianity in France and in the world."
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."
The President of the European Commission Jean-Claude Juncker described the fire as a "horror".
"I share the emotion of the French nation which is also ours," he said in a press release.
President of the European Council Donald Tusk said "we are all with Paris today".
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!"
London's Westminster Abbey said it is "devastated for our friends" at Notre-Dame.
"You are in our thoughts and prayers tonight," a post from its official Twitter account read.
The Archbishop of Canterbury Justin Welby tweeted: "Tonight we pray for the firefighters tackling the tragic #NotreDame fire - and for everyone in France and beyond who watches and weeps for this beautiful, sacred place where millions have met with Jesus Christ. Nous sommes avec vous."
Mayor of London. Sadiq Khan described the scenes as "heartbreaking.
He tweeted: "London stands in sorrow with Paris today, and in friendship always."
Labour MP Yvette Cooper said she is in Paris and saw the spire of Notre-Dame fall, but "can't bear to watch any more".
"Have just come away from the bank of the Seine after the spire fell as I can't bear to watch any more," she tweeted.
"Fearful for anyone close to the flames, and aghast that centuries of history & beauty could disappear into smoke so fast."
Paris police urged people to "avoid the area and facilitate the passage of emergency vehicles".
The cathedral, which dates back to the 12th century and is famous for featuring in Victor Hugo’s classic novel the Hunchback of Notre-Dame, attracts millions of tourists every year.