Firefighters were working from the inside and outside with hoses against the massive blaze.
Thousands of onlookers lined bridges over Seine and along its embankments, held at a distance by a police cordon. "Emotion of a whole nation", French President tweeted, noting further that he was "sad to see a part of us burn this evening".
The Metropolitan Museum of Art in NY also expressed its condolences, tweeting, "The Met and the collective cultural community are watching in shock and horror at the fire engulfing Notre-Dame Cathedral". He also said that several paintings were saved, adding that "some were too big" to take down.
Macron said a national fundraising campaign to restore Notre Dame would be launched Tuesday and he called on the world's "greatest talents" to help.
On Twitter, Trump suggested using planes to drop water on the flames.
The Prime Minister of Canada tweeted in French and English saying the fire was "heartbreaking".
The cause is not yet clear, but officials say that it could be linked to renovation work. The city's mayor, Anne Hidalgo, said the significant collection of art work and holy objects inside the church had been recovered.
On Monday, footage captured a massive fire atop Notre Dame Cathedral, Paris's most famous building. First immortalized in Victor Hugo's book "The Hunchback of Notre Dame", the building has always been a subject of fascination of historians and art lovers, drawing roughly 12 million tourists every year.
He extended "thoughts for all the Catholics and all the French".
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Paris' Notre Dame Cathedral, which caught fire on Monday, had reached a "tipping point" and required an expensive makeover after deteriorating for years, experts had previously warned.
"I will tell you, the fire that they are having at the Notre Dame cathedral is something like few people have witnessed".
"I've visited it so many times, but it will never be the same". "It's a symbol of Paris".
One report states that a renovation worker at the cathedral claims the fire was set intentionally.
It is renowned for its rib vaulting, flying buttresses and stunning stained glass windows, as well as its many carved stone gargoyles.
The US president has tweeted that "flying water tankers" should be used to combat the blaze, which has devastated the 850-year-old cathedral. "So disgusting to watch the massive fire at Notre Dame Cathedral in Paris". It was the site of a Huguenot riot in the 1500s and was damaged in the French Revolution in 1793, when statues mistakenly believed to be of French kings were beheaded.
The wood-and-lead spire was built during that restoration, according to the cathedral's website.
"Notre-Dame consumed by flames".