Firefighters struggled to extinguish the blaze. — AFP Photo

PARIS — Crowds of stunned Parisians and tourists -- some crying, others offering prayers -- watched in horror in central Paris on Monday night as firefighters struggled for hours to extinguish the flames engulfing the Notre-Dame Cathedral.

Flames ravaging the roof illuminated the outline of the monument's two square towers in a fiery glow, and were reflected in the waters of the Seine. 

Along the Pont au Change bridge, which connects the Ile de la Cite with the Right Bank, the atmosphere was one of a vigil as hundreds of people watched in hushed silence as smoke rose into the night sky.

Many were quietly singing an Ave Maria in Latin, including Stephane Seigneurie, 52, who said he has lived in Paris for the past 25 years. 

"I come often, and go in even where there's no mass because it's an extraordinary place, entwined in the history of France," he said.

"Politically, intellectually and spiritually, it's a symbol of France."

When Seigneurie says that he's very sad, an elegant woman with dark bobbed hair who is crying whispers to him, "We have to pray."

Jeanne Duffy, 62, had travelled from New York to Paris with her twin daughters to see her nephew run the Paris marathon on Sunday.

The girls had wanted to climb the church's towers Monday evening but at the last minute the three decided to go to Disneyland Paris instead.

"We were heartbroken because as New Yorkers we've been through this," Duffy said, referring to the September 11, 2001 attacks which destroyed the twin towers of the World Trade Center.

"In terms of heritage this is much worse. This is a world treasure.

Everyone knows Notre-Dame," she said.

Tragedy

Smoke billows as flames burn through the roof of the Notre-Dame de Paris Cathedral on April 15, 2019, in the French capital Paris. - A huge fire swept through the roof of the famed Notre-Dame Cathedral in central Paris on April 15, 2019, sending flames and huge clouds of grey smoke billowing into the sky. The flames and smoke plumed from the spire and roof of the gothic cathedral, visited by millions of people a year. — AFP Photo

Gasps and cries of "Oh my god" erupted at 7:50 pm (1750 GMT) when the top portion of the church's spire came crashing down into an inferno that had spread to the entire roof.

More gasps came a few seconds later when the rest of the spire collapsed, caught on the cameras of thousands of mobile phones.

"Paris is disfigured. The city will never be like it was before," said Philippe, a communications worker in his mid-30s, who had biked over after being alerted of the fire by a friend.

"I'm a Parisian, my father was a Parisian, my grandfather as well -- this was something we brought our sons to see," he said. "I won't be showing this to my son."

"It's a tragedy," he added. "If you pray, now is the time to pray."

Police cleared pedestrians away from the two islands in the river Seine, including the Ile de la Cite which houses the soaring Gothic church, one of Europe's best-known landmarks.

But throngs of onlookers remained behind police cordons on the stone bridges leading to the islands and along the banks of the Seine river as darkness fell.

"It's finished, we'll never be able to see it again," said Jerome Fautrey, a 37-year-old who had come to watch.

History up in smoke

Sam Ogden, 50, had arrived from London on Monday with her husband, their two teenaged sons, and her mother. They had come to Paris specifically to see Notre-Dame, part of a world tour over years to see historic sites.

"This is really sad -- the saddest thing I've ever stood and watched in my life," Ogden said.

She said the fire looked tiny at the beginning, "then within an hour it all came down."

Emilia Freitas, a French teacher from Portugal, was visiting Paris with her husband and daughter, who is studying architecture.

"We were very sad because it's a very important monument, and also worried because many things have happened in Paris lately," she added, referring in particular to the jihadist terror attacks that have struck the city since 2015.

Miguel-Angel Godia, his wife Esther Fajardo and their daughter Raquel, 10 -- who had seen the animated "Hunchback of Notre-Dame" film -- had planned on visiting the church for the first time Tuesday.

"It's a real shame... it's something so immense, so emblematic," said Esther, wiping tears from behind her glasses with the tip of a scarf.

Anxious passers-by watched the landmark cathedral burn. — AFP Photo

Macron vows to rebuild

French President Emmanuel Macron on Monday vowed to rebuild Cathedral after the colossal fire that wiped out centuries of heritage.

Macron expressed relief that "the worst had been avoided" in a fire that risked bringing the entire edifice down and left France in shock over the damage to a building described as the soul of the nation.

But the blaze destroyed the roof of the 850-year-old UNESCO world heritage landmark, whose spectacular Gothic spire collapsed before the eyes of horrified onlookers on a previously pristine early spring evening.

As darkness fell, some 400 firefighters then battled to control the blaze and save at least its iconic front towers. They finally gained the upper hand as midnight approached.

Paris fire brigade chief Jean-Claude Gallet said "we can consider that the main structure of Notre-Dame has been saved and preserved" as well as the two towers.

Deputy Interior Minister Laurent Nunez, also present at the scene, said that for the first time "the fire had decreased in intensity" while still urging "extreme caution".

France is Notre Dame

"Notre-Dame survived all the wars, all the bombardments. We never thought it could burn. I feel incredibly sad and empty," Stephane Seigneurie, a consultant who joined other shocked onlookers in a solemn rendition of "Ave Maria" as they watched the fire from a nearby bridge.

Gasps and cries of "Oh my god" erupted around an hour after the fire first broke out when the top portion of the church's spire came crashing down.

"We have been dealt a knockout blow," a stricken-looking Paris Archbishop Michel Aupetit told reporters.

The cause of the blaze was not immediately confirmed. The cathedral had been undergoing intense restoration work which the fire service said could be linked to the blaze.

French prosecutors said it was being treated as an "involuntary" fire, indicating that foul play was ruled out for now.

Historians expressed incredulity at the collapse of a building that has been a symbol of France for almost a millennium and withstood war and revolution.

"If Paris is the Eiffel Tower then France is Notre Dame. It's the entire culture, entire history of France incarnated in this monument," Bernard Lecomte, a writer and specialist in religious history told BFM TV.

Deputy Paris mayor Emmanuel Gregoire told the channel that workers were scrambling "to save all the artworks that can be saved." Officials later said teams had managed to salvage an unknown quantity of the cultural treasures.

A man is painting Notre-Dame Cathedral in July 1945 on the banks of the Seine river, near the Ile de la Cité in Paris. - A huge fire swept through the roof of the famed Notre-Dame Cathedral in central Paris on April 15, 2019. French President Emmanuel Macron vows to rebuild Notre-Dame after devastating fire. — AFP Photo

Emotion of a nation

Macron had earlier cancelled a major televised policy speech he was due to give on Monday evening to respond to months of protests, and instead headed to the scene in person.

Visibly emotional, he said while the "worst had been avoided" and the facade and two towers saved "the next hours will be difficult".

"And we will rebuild Notre-Dame because it is what the French expect," he said, describing Notre Dame as "the epicentre of our life" and the cathedral of "all the French", whether religious or not.

France's billionaire Pinault dynasty immediately pledged 100 million euros for the effort.

German Chancellor Angela Merkel called Notre-Dame cathedral a "symbol of European culture" as the blaze raged.

The Vatican on Monday expressed its "incredulity" and "sadness", expressing "our closeness with French Catholics and with the Parisian population."

Water bombers not used

There was no immediate indication of any casualties in the blaze. 

US President Donald Trump in a tweet said it was "horrible" to watch the fire but caused controversy by offering advice on how to put it out.

"Perhaps flying water tankers could be used to put it out. Must act quickly!" he said.

But France's civil security service, which oversees crisis management in the country, tweeted back at Trump that the use of water-bombing aircraft was not being considered.

"If used, (this) could lead to the collapse of the entire structure of the cathedral," it said.

Will never be the same

The cathedral was located at the centre of the French capital in the Middle Ages and its construction was completed in the mid-12th century after some 200 years of work.

During the French Revolution in the 18th century, the cathedral was vandalised in widespread anti-Catholic violence: its spire was dismantled, its treasures plundered and its large statues at the grand entrance doors destroyed.

It would go on to feature as a central character in a Victor Hugo novel published in 1831, "The Hunchback of Notre-Dame" and shortly afterwards a restoration project lasting two decades got under way, led by architect Eugene Viollet-le-Duc.

It would survive the devastation of two world conflicts in the 20th century and famously rang its bells on August 24, 1944, the day of the Liberation of Paris from German occupation at the end of the World War II.

"Paris is disfigured. The city will never be like it was before," said Philippe, a communications worker in his mid-30s, who had biked over to the scene after being alerted of the fire by a friend.

Jacky Lafortune, a 72-year-old artist and self-described atheist stood forlornly on the banks of the River Seine staring at the cathedral.

Comparing the mood in the French capital to the aftermath of a terror attack he said: "But this stirs much deeper emotions because Notre-Dame is linked to the very foundations of our culture."

Some treasures saved

Around 400 French firefighters faced a rapidly spreading fire at Notre-Dame as they arrived on the scene Monday evening, but appeared to have prevented the complete destruction of the cathedral and managed to rescue some of its priceless artifacts by rushing inside, officials said.

Dozens of fire engines and at least 18 high-pressure aerial hoses were used to contain the blaze.

"It spread extremely quickly on the roof," Paris deputy mayor Emmanuel Gregoire told the BFM news channel.

Wooden roof beams dating from the 12th century were to blame, but the first-responders had made "saving as many artworks as possible" a priority, Gregoire explained.

Some teams had managed to salvage an unknown quantity of its cultural treasures, he and the head of the Paris fire service said.

The Holy Crown of Thorns and a sacred tunic worn by 13th-century French king Louis, two irreplaceable artifacts, had been rescued, the cathedral rector Patrick Chauvet said.

President Macron paid tribute to the efforts of the fire service as he visited the scene, sending the "thanks on behalf of the whole nation."

He praised their "extreme courage, their great professionalism and lots of determination by their commanders." — AFP