Investigators are treating the fire that engulfed Paris' Notre Dame Cathedral as an accident for now, the local prosecutor's office said Monday evening.

Paris police will investigate the disaster as "involuntary destruction caused by fire" and have ruled out arson and potential terror-related motives for starting the blaze, officials said.

The prosecutors' statement came moments after a government official and the Paris fire chief said they were optimistic that the cathedral's world-famous bell towers had been saved from the fire, and that the main structure of the building remained intact. It was a rare piece of good news on a night that left thousands in the streets of the French capital and millions around the world looking on in shock as one of the world's great religious, cultural and historic landmarks was consumed by flames.

Fire chief Jean-Claude Gallet confirmed that firefighters had managed to stop the fire spreading to the northern belfry, the stomping ground of the fictional hunchback Quasimodo in Victor Hugo's 1831 novel "The Hunchback of Notre Dame." He added that two-thirds of the cathedral's roof "has been ravaged" and confirmed that one firefighter was injured, the only known injury from the fire.

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. Gallet said firefighters would keep working overnight to cool down the building.

Smoke seen in the interior of Notre Dame cathedral in Paris after the fire broke out Monday. (Philippe Wojazer/Pool via AP)

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The blaze collapsed the cathedral's spire, which had been shrouded in scaffolding as part of a 6 million-euro ($6.8 million) renovation project on the spire and its 250 tons of lead. French media quoted the Paris fire brigade as saying the fire was "potentially linked" to that construction. 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

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.

French President Emmanuel Macron, who canceled a planned address to the nation about the ongoing "yellow vest" crisis, rushed to the scene and straight into meetings at the Paris police headquarters nearby. After emerging to face a throng of reporters, Marcon vowed the cathedral would be rebuilt and announced the launch of an international fundraising drive to begin raising the millions of dollars necessary to restore the building to its former glory.

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"We will rebuild this cathedral," said Macron, who added that "The worst has been avoided although the battle is not yet totally won." The French leader also praised the "courage" and "great professionalism" of firefighters who battled the blaze for approximately four hours.

Paris Archbishop Michel Aupetit invited priests across France to ring church bells in a call for prayers for the beloved Paris 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 river, its architecture is famous for, among other things, its many gargoyles and its iconic flying buttresses.

Among its most celebrated art: 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. Paris Mayor Anne Hidalgo tweeted early Tuesday that firefighters and "municipal agents" had rescued several prominent items from the collection of artwork and holy objects by passing them along what she called "a tremendous human chain."

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"The Crown of thorns, the tunic of Saint Louis and several other major works are now in a safe place," Hidalgo tweeted. It was not clear how many items had been destroyed or damaged.

French historian Camille Pascal told the BFM broadcast channel the fire marked "the destruction of invaluable heritage."

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"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."

Fox News' Lucia Suarez Sang and The Associated Press contributed to this report.