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A massive fire engulfed the upper reaches of Paris' soaring Notre Dame Cathedral as it was undergoing renovations Monday, threatening one of the greatest architectural treasures of the Western world as tourists and Parisians looked on aghast from the streets below.

The blaze collapsed the cathedral's spire and spread to one of its landmark rectangular towers, but Paris fire chief Jean-Claude Gallet said the church's structure had been saved after firefighters managed to stop the fire spreading to the northern belfry. The 12th-century cathedral is home to incalculable works of art and is one of the world's most famous tourist attractions, immortalized by Victor Hugo's 1831 novel "The Hunchback of Notre Dame."

The exact cause of the 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. The Paris prosecutors' office ruled out arson and possible terror-related motives, and said it was treating it as an accident.

As the spire fell, the sky lit up orange and 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 church, watching in shock as acrid smoke rose in plumes. Speaking alongside junior Interior minister Laurent Nunez late Monday, police chief Jean-Claude Gallet said "two thirds of the roofing has been ravaged." Gallet said firefighters would keep working overnight to cool down the building.

Late Monday, signs pointed to the fire nearing an end as lights could be seen through the windows moving around the front of the cathedral, apparently investigators inspecting the scene. Remarkably, only one of the about 400 firefighters who battled the blaze was injured, officials said.

The fire came less than a week before Easter amid Holy Week commemorations. As the cathedral burned, Parisians gathered to pray and sing hymns outside the church of Saint Julien Les Pauvres across the river from Notre Dame while the flames lit the sky behind them. Paris Archbishop Michel Aupetit invited priests across France to ring church bells in a call for prayers.

French President Emmanuel Macron was treating the fire as a national emergency, rushing to the scene and straight into meetings at the Paris police headquarters nearby. He pledged to rebuild the church and said would seek international help to do so.

"The worse has been avoided although the battle is not yet totally won," the president said, adding that he would launch a national funding campaign on Tuesday and call on the world's "greatest talents" to help rebuild the monument.

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 river, its 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.

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."

Paris Mayor Anne Hidalgo said in a Twitter message that Paris firefighters were 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 the Paris diocese.

Reactions from around the world came swiftly including from the Vatican, which released a statement expressing shock and sadness for the "terrible fire that has devastated the Cathedral of Notre Dame, symbol of Christianity in France and in the world."

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."

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Associated Press writers Elaine Ganley and Sylvie Corbet contributed.