Video report by ITV News Europe Editor James Mates
French President Emmanuel Macron has said he wants to see Paris's fire-ravaged Notre Dame cathedral rebuilt within five years.
Millions of Euro have been pledged towards the rebuilding, with some of the country's most influential business people promising cash toward the restoration.
Whilst the damage is significant, parts of the cathedral have been saved.
Firefighters worked through the night and managed to save the treasured stone facade of the Gothic cathedral and two bell towers from complete destruction.
However, the spire and roof collapsed and was sent crashing to the crowd, in front of shocked Parisians, who mourned the destruction of the iconic landmark.
The blaze started at around 7pm (6pm BST) and was brought under "complete control" nine hours later.
Paris firefighters said at around 10am (9am BST) on Tuesday the fire had been "fully extinguished", despite initial reports it would take days to put out.
It's now emerged investigators have questioned about 30 people following the fire.
Most of them were employees working on the renovation of the monument, an unnamed judicial official has said.
The official said the cathedral's fire alarms sounded twice on Monday evening.
The first time, some people, including a fire official permanently working on the site, went to check under the roof and saw nothing.
The second time it was already too late because the fire was too strong, the official said.
At Westminster Abbey, bells tolled at 5.43pm to mark the moment the Notre Dame fire started, and show Britain's solidarity with France.
The prime minister said bells at churches and cathedrals across England will be rung on Thursday.
"Notre Dame is one of the most beautiful buildings in the world - a symbol of France and the French people, and cherished across the globe," Mrs May said.
"The images of destruction we saw last night were truly heart-rending."
Mrs May said the UK will offer any support France needs once the damage has been assessed.
In Paris, people have gathered to hold a vigil in honour of the building. Parisians sang hymns as they walked through the city's streets.
Footage from inside the cathedral shows the smoldering pile of what appeared to be the charred remains of the roof and spire in front of the altar, while a cross escaped destruction and gloowed from within the gloom.
Inside the cathedral show several pieces of wooden furniture, including chairs and benches can be seen, seemingly intact. But a gaping hole in the nave's roof, with a pile of burnt debris lying underneath, means a repair job will be prolonged and expensive.
French prosecutors are investigating an "accidental destruction by fire" but said there is no evidence the fire had been started on purpose.
The cause of the fire is still not known, and could be linked to the ongoing renovation work of the cathedral.
Fifty people are working on a "long" and "complex" investigation into the cause, Paris prosecutor Remy Heitz told reporters.
ITV News Correspondent Emma Murphy spoke to a local woman about the impact the fire has had.
"You're just watching your main woman of the town dying in front of your eyes and you can't do anything," she said.
"It's not just about religion, so it's kind of a big house for everyone and to see that house burning, it's a real strong image to see."
French President Emmanuel Macron visited the scene of the devastation on Monday evening, after cancelling a national address.
He announced the launch of a national subscription to rebuild the national monument and has cancelled his European election campaign in the wake of the fire.
"Notre Dame is our history, our imagination, where we've lived all our great moments, and is the epicentre of our lives," Mr Macron said.
"It's the story of our books, our paintings. It's the cathedral for all French people, even if they have never been. But it is burning and I know this sadness will be felt by all of our citizens."
Crowds reacted in shock at the moment when the spire collapsed amid the raging fire.
The first harrowing images from within the fire-ravaged cathedral began to emerge as firefighters brought the blaze under control.
Crowds gathered outside the cathedral to see their city's iconic sight burn, with tears in their eyes they sang.
Some of the cathedral's most precious objects, including a relic purported to be the crown of thorns worn by Jesus Christ on the cross, were whisked away to a secure facility.
French culture minister Franck Riester said artworks will be transferred to the Louvre Museum.
He said: "First of all the treasures, the most precious ones, were saved last night and stored at the Hotel de Ville in Paris, and I'd like to thank the town hall of Paris, and also the teams of ministry of culture, the fire officers and also everyone who really tried to save the crown (of thorns) and various other treasures."
The crown of thorns is one of Notre Dame's most precious treasures and the Roman Catholic Church said was placed on Jesus' head before his crucifixion.
There were hopes the three famous rose windows survived devastation, while the bells that have rung out at key moments in France's history were thought to be safe.
(£173 million) towards the effort to rebuild the cathedral, Billionaire French fashion mogul Francois-Henri Pinault pledged 100 million euros (£86 million) and Total, the French oil company, has contributed 100 million euros.
Voices from across France have come together to support the community around the Notre Dame. Paris's Great Mosque has offered its condolences, saying its worshipers should have contribute to the rebuilding of the cathedral. A delegation from the mosque visited the burnt out shell of the building on Tuesday afternoon.
The world reacted with shock and grief at the devastation of the internationally renowned cathedral.
The Queen said she was "deeply saddened" by the Notre Dame fire and extended her thoughts and prayers those who worship at the landmark and "all of France at this difficult time".
The Prince of Wales said he and the Duchess of Cornwall were "utterly heartbroken" and described the events as a "shattering tragedy, the unbearable pain of which we all share".
The Pope and the Vatican expressed "shock and sadness" over news of the fire.
Pope Francis tweeted: "Today we unite in prayer with the people of France, as we wait for the sorrow inflicted by the serious damage to be transformed into hope with reconstruction. Holy Mary, Our Lady, pray for us. #NotreDame"
Prime Minister Theresa May sent her wishes to the French capital and said her "thoughts are with the people of France" and the emergency services.
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.
He added a message of support in French: "Nous sommes avec vous," meaning we are with you.