PARIS (AP) — A massive fire engulfed the roof of Notre Dame Cathedral in the heart of the French capital Monday, toppling its spire and sending thick plumes of smoke high into the blue sky as tourists and Parisians watched in horror from the streets below.

A spokesman said the entire wooden frame of the cathedral would likely come down, and that the vault of the edifice could be threatened too.

“Everything is burning, nothing will remain from the frame,” Notre Dame spokesman Andre Finot told French media. The 12th-century cathedral is home to incalculable works of art and is one of the world’s most famous tourist attractions.

The cause of the catastrophic blaze was not known, but French media quoted the Paris fire brigade as saying the fire is “potentially linked” to a 6 million-euro ($6.8 million) renovation project on the church’s spire and its 250 tons of lead. Prosecutors opened an investigation as Paris police said there were no reported deaths.

Flames shot out of the roof behind the nave of the cathedral, among the most visited landmarks in the world. Hundreds of people lined up bridges around the island that houses the cathedral, watching in shock as acrid smoke rose in plumes.

French President Emmanuel Macron postponed a televised speech to the nation because of the stunning blaze and was going to the cathedral himself.

Paris deputy mayor Emmanuel Gregoire said emergency services are trying to salvage the famed art pieces stored in the cathedral.

Built in the 12th and 13th centuries, Notre Dame is the most famous of the Gothic cathedrals of the Middle Ages as well as one of the most beloved structures in the world. Situated on the Ile de la Cite, an island in the Seine, the cathedral’s architecture is famous for, among other things, its many gargoyles and its iconic flying buttresses.

Among the most celebrated artworks inside are its three stained-glass rose windows, placed high up on the west, north and south faces of the cathedral. Its priceless treasures also include a Catholic relic, the crown of thorns, which is only occasionally displayed, including on Fridays during Lent.

The cathedral was immortalized in Victor Hugo’s “The Hunchback of Notre Dame,” published in 1831, and has long been a subject of fascination in popular culture as well as the traditional art world.

French historian Camille Pascal told BFM broadcast channel the blaze marked “the destruction of invaluable heritage.”

“It’s been 800 years that the Cathedral watches over Paris”, Pascal said. “Happy and unfortunate events for centuries have been marked by the bells of Notre Dame.”

He added: “We can be only horrified by what we see.”

Associated Press reporters at the scene saw massive plumes of yellow brown smoke filling the air above the Cathedral and ash falling on the island that houses Notre Dame and marks the center of Paris. As the spire fell, the sky lit up orange.

Paris Mayor Anne Hidalgo is in despair at the “terrible fire.” Hidalgo said in a Twitter message that Paris firefighters are still trying to limit the fire and urged Paris citizens to respect the security perimeter that has been set around the cathedral.

Hidalgo said Paris authorities are in touch with Paris diocese.

Reactions from around the world came swiftly.

In Washington, Trump tweeted: “So horrible to watch the massive fire at Notre Dame Cathedral in Paris” and suggested first responders use “flying water tankers” to put it out.

Cardinal Timothy Dolan, the archbishop of New York, said he was praying “to ask the intercession of Notre Dame, our Lady, for the Cathedral at the heart of Paris, and of civilization, now in flames! God preserve this splendid house of prayer, and protect those battling the blaze.”