Updated at 5:40 p.m. ET

Notre Dame Cathedral, one of the world's most famous churches, erupted in flames Monday in Paris — and though its survival remains in question, authorities expressed hope.

The commander of the Paris firefighter brigade said Monday night that the cathedral's main structure and two towers of the cathedral had been saved.

Flames leaped through the roof and dark smoke billowed into the sky on Monday afternoon. Observers gasped as the spire fell. The roof collapsed shortly afterward.

"Pray," Archbishop of Paris Michel Aupetit wrote. "If you wish, you can ring the bells of your churches to invite to prayer."

As night fell on Paris and the fire continued to burn, people kneeled and sang the "Ave Maria" as they watched the fire burn.

Reuters reports that one fireman was injured seriously in the blaze.

The fire broke out during Holy Week for the world's Roman Catholics. At least four masses a day take place at the cathedral. Deputy mayor Emmanuel Gregoire said authorities were trying to save priceless art and relics from the structure's interior, according to the Associated Press.

The Paris prosecutor's office says it has opened an investigation into the fire, but that initial indications suggest it was started accidentally. The cathedral closed at 6:45 p.m. local time, and the fire broke five or six minutes later, France 24 reported.

Police in Paris asked people to avoid the area so emergency vehicles could pass. The area was evacuated and a shelter was set up for residents who live near the cathedral.

NPR's Eleanor Beardsley was at the scene, standing with Parisians who watched, stunned.

"There's kind of a silence here," Beardsley said. "People are not screaming, they're just watching it burn and people are in a state of shock. Outside in the periphery, it's pandemonium in the city. They've stopped the trains from going by here, they're stopping traffic, police everywhere. But in a tighter circle around the cathedral we're just watching it burn."

The smoke billowed, darkening the sky for miles.

Two thousand people were inside when the fire broke out, but none were injured. "They did get some artwork out, but it's going to be cataclysmic damage," Beardsley reported.

Paris Mayor Anne Hidalgo called the fire "terrible" and said firefighters are trying to control the flames. The city's deputy mayor said the cathedral had suffered "colossal damages."

French President Emmanuel Macron canceled an address to the nation that was planned. "Notre-Dame de Paris is in flames," he tweeted. "Emotion of an entire nation. Thinking of all Catholics and all French people. Like all our compatriots, I am sad this evening to see this part of us burn."

The structure has been undergoing a $6.8 million renovation project, including work on its spire.

Located on the Île de la Cité in the middle of the Seine River, the soaring Gothic structure dates to the 13th century and was completed in the 15th century. The city said that it had closed all entrances to the isle.

The Notre Dame celebrated its 850th birthday in 2013. It attracts 12 million visitors each year. It is the most visited monument in France, ahead of the Eiffel Tower, according to the cathedral.Victor Hugo used the cathedral's gargoyles and flying buttresses as the setting for his 1831 novel The Hunchback of Notre Dame.

Many observers wondered why airplanes were not dropping water on the inferno, as firefighters do to combat forest fires. But the cathedral's urban setting makes such a mission too dangerous: it would involve dropping perhaps 6 tons of water, an unsafe proposition in a densely populated city. The intervention could destroy the cathedral in the effort to save it.

France's civil defense agency tweeted that a large drop of water could cause the entire structure to collapse, and endanger the firefighters below.

Caroline Bruzelius, an art historian from Duke University who has studied Notre Dame for years, tells NPR that above the limestone vaults is wood that amounts to a "forest of timber" some 50 or 60 feet high that support the roof. "This timber is old, it's very dry, and very flammable."

She offered some sense of the damage the limestone vaults could have sustained: "When it's exposed to fire, stone is damaged. It doesn't actually burn but it loses its surface," she says. "It chips, and it's no longer structurally sound."

The Vatican issued a statement that "The Holy See has seen with shock and sadness the news of the terrible fire that has ravaged the Cathedral of Notre Dame, symbol of Christianity, in France and in the world." It expressed closeness with French Catholics and the people of Paris, and said the Vatican prays for the firefighters.

At an event in Minnesota, President Trump called the Notre Dame "one of the great treasures of the world."

"It looks like its burning very badly, it looks like it's burning to the ground," Trump said, causing the crowd to gasp.

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