By Eric Levenson, Melissa Bell and Emanuella Grinberg, CNN

(CNN) -- Parisians raised their voices in song Monday night outside Notre Dame Cathedral as firefighters battled a massive blaze threatening one of France's most revered attractions.

The fire burned for several hours Monday, causing the collapse of the iconic spire towering above the cathedral.

Consumed by flames, the spire leaned to one side and fell onto the burning roof as horrified onlookers watched.

By late Monday night, both of the cathedral's towers were safe, said Laurent Nunez, secretary to the interior minister.

"The fire is now weaker. We are now in a time of cooling but both towers of the cathedral are safe," Nunez told reporters at the scene.

At least one firefighter injured

For much of the day, flames and thick plumes of smoke billowed from the cathedral, including the bell tower and the spire. Rescue workers rushed to evacuate artworks.

It was not immediately known what caused the fire. The cathedral is surrounded by scaffolding amid construction work. Gallet said the initial call to emergency services notified authorities of a fire in the attic of the cathedral.

About 400 firefighters were mobilized to deal with the blaze, the French Interior Ministry said. Firetrucks had difficulty accessing the scene, located on an island in Paris, amid the daily afternoon rush hour. Police urged the public to avoid the area.

Firefighters atop cherry picker cranes sprayed water onto the church in an attempt to calm the flames.

One firefighter was seriously injured, Paris Fire Brigade Commander General Jean-Claude Gallet said.

'Madness'

The fire, just days before Easter, was met with horror by Parisians and tourists.

Thibaud Binétruy, who lives in Paris, said he saw the smoke while walking home with a colleague and was gripped by the sight.

"When the spire fell, the crowd reacted with ohhh and ahh, but I guess most of them were just shocked silently," Binétruy said. "It's awful to see such a symbol disappearing in front of you. It's been there for so many years and in a few minutes half of it disappeared ... crazy. Paris without Notre Dame, madness."

Patryk Bukalski was at a café near the cathedral when he started smelling smoke.

"A bartender said the Notre Dame is on fire, so I left and saw this horrible picture," he said over Instagram.

He said people are standing outside, watching and crying.

"It's horrible. It's a symbol of Paris," Bukalski said. "It's just sad. I don't know what more to say."

Another witness, Anne Marie, spoke with tears in her eyes.

"In Paris, it's a monumental symbol -- every person with different religions are really moved and saddened," she said. "Paris without the cathedral is not Paris anymore."

The Vatican said the Holy See learned with "shock and sadness the news of the terrible fire that has devastated the Cathedral of Notre Dame de Paris, symbol of Christianity, in France and in the world."

"We express our closeness to the French Catholic and to the people of Paris. We pray for the firefighters and for all those who are doing everything possible to face this dramatic situation," the Vatican said.

A symbol of France

Notre Dame's foundation stone was laid in 1163 by Pope Alexander III, and the cathedral was completed in the 13th century. Today, with its towers, spire, flying buttresses and stained glass, Notre Dame is considered a feat of architecture as well as a major religious and cultural symbol of France.

Located in Île de la Cité, a small island in the middle of the city, the cathedral is one of Paris' most popular attractions, drawing an estimated 13 million visitors a year.

Even as it fell into disrepair over the centuries, it was the site of Napoleon Bonaparte's coronation as emperor in 1804. The central spire was built in the 19th century amid a broad restoration effort, partly buoyed by the success of Victor Hugo's novel "The Hunchback of Notre-Dame" in 1831.

The cathedral also houses the grand organ, one of the world's most famous musical instruments, as well as the Crown of Thorns, a relic of the passion of Christ.

French President Emmanuel Macron postponed a planned speech Monday night because of the Notre Dame fire, a spokesperson for the Elysee Palace said.

"Notre Dame of Paris in flames. Emotion for a whole nation. Thoughts for all Catholics and for all French. Like all our countrymen, I'm sad tonight to see this part of us burn," Macron said on Twitter.

Macron arrived at the scene, accompanied by Paris Mayor Anne Hidalgo, French Prime Minister Edouard Philippe and first lady Brigitte Macron, as well as the Paris prosecutor, who has opened an investigation into the fire.

Cardinal Daniel DiNardo, the president of the US Conference of Catholic Bishops, said Notre Dame "is not only a majestic church, it is also a world treasure," he said.

"Noble in architecture and art, it has long been a symbol of the transcendent human spirit as well as our longing for God," he said.

US President Donald Trump said it was a "terrible, terrible fire" and lamented the damage done to the historic cathedral.

"That's beyond countries, that's beyond anything," he said. "It's a part of our culture."