PARIS — The world-renowned Notre Dame cathedral in Paris was devastated by a massive fire Monday as thousands of Parisians looked on in horror.

The fire broke out during the afternoon rush hour in Paris, making it difficult for firefighters to get to the cathedral.

A French Interior Ministry official had told the Associated Press that the iconic building may not be saved, though firefighters later said the structure of Notre Dame had been saved from total destruction, and that there was optimism that the main towers could be preserved.

An orange glow could be seen in the heart of the cathedral more than an hour after firefighters battled back the high flames.

"The bells of the cathedral are melting," France Ambassador to the U.S. Gerard Araud told CNN.

Two thirds of the cathedral's roof was destroyed, and one firefighter was seriously injured, CNN reported. The interior of the cathedral was gutted.

"It’s a loss for the heart of France," UCF professor and historian Dr. Amelia Lyons said. "There are a handful of structures that we all know when we think of Paris and we think of France: We think of the Eiffel Tower, Versailles, L’Arc De Triomphe. Notre Dame is one of the most visited site in the city."

At the height of the fire, thick yellow and black smoke and bright orange flames billowed from the roof of the cathedral.

"Just thousands of people out here, and the French people are really upset because this one of the most unique and reverent places inside of France and inside of Paris," said Steve Fernandez, who lives in Paris. "Everybody its calm, but it's crazy. Everyone is in shock that it’s happening.”

Parisians and tourists watched in shock, some through tears, as the iconic cathedral burned into the night. Others gathered to pray and sing hyms.

The massive fire raged out of control for more than 15 to 20 minutes and took down the iconic spire, which had been undergoing renovation. Scaffolding had been up around it.

Crews also worked to save famous pieces of art, stained glass, and sculptures from inside Notre Dame.

Paris prosecutors have opened an investigation into the fire, and preliminarily, they think the cause of the fire was accidental.

On Twitter, French President Emmanuel Macron said the nation was hurting.

"Like all my fellow citizens, I am sad to see this part of us burn tonight," he said.

He said "the worst has been avoided," and "we will rebuild this cathedral together," announcing a national fundraising effort toward the gutted landmark.

Notre Dame, in the middle of Paris, is a huge tourist attraction right on the River Seine and is one of the most visited sites in France. The site dates back almost 1,000 years, and the cathedral is still in use as a church today.

"The horrific fire that is engulfing the Cathedral of Notre-Dame de Paris is shocking and saddens us all, for this particular cathedral is not only a majestic Church, it is also a world treasure," Cardinal Daniel DiNardo, the president of the U.S. Conference of Catholic Bishops. "Our hearts go out to the Archbishop and the people of Paris, and we pray for all the people of France, entrusting all to the prayers and intercession of the Mother of God, especially the firefighters battling the fire."

It's unclear how many people were inside. Officials told the news agency France 24 that no one was killed.

Fire unfolds on social media

Notre Dame cathedral is a major tourist attraction in the heart of Paris. It's still used as a church. (Kate Fox/Spectrum News)
Notre Dame cathedral is a major tourist attraction in the heart of Paris. It's still used as a church. (Kate Fox/Spectrum News)