Flames cause 'colossal damages' to Notre Dame Cathedral in Paris, Macron says 'we will rebuild'

A fire has engulfed the Notre Dame Cathedral in Paris, one of the city's most iconic sights.

The fire began at 6:50 p.m. local time Monday, the Paris fire department said, and continued to burn hours later.

A French firefighter official told reporters the main structure of the cathedral has been saved and the two iconic main towers are intact and have been saved as well.

French Mayor Anne Hidalgo said that authorities were evacuating people from buildings in the area -- including homes close to the cathedral.

(MORE: Notre Dame: The national and architectural significance of the historic cathedral)

A spokesperson for the cathedral described the damage to French media as "colossal."

"Everything is burning, nothing will remain from the frame," the spokesperson said.

First responders are currently trying to salvage priceless art stored inside the cathedral, said Paris Deputy Mayor Emmanuel Gregoire. Last week, the statues of the apostles were removed for restoration, The Associated Press reported.

Images on social media show smoke and flames shooting from the top of Notre Dame as massive plumes of smoke billow into the sky above.

One video shows a spire toppling over as a result of the flames.

The fire may be connected to the recent restoration of the church, officials said. The restoration work costs 120 million euros, the Notre Dame Paris office told ABC News, adding that a fire alarm test conducted last week went well.

The roof has completely collapsed, witness John Dickas, an American professor living in Paris, told ABC News.

Large crowds have begun to gather around the scene. Some members of the crowd began to sing hymns as the cathedral burned.

The cathedral was built around 1260 AD and towered over the city of Paris for centuries. Much of the facade and interior are true to their original designs.

French President Emmanuel Macron expressed sadness at the sight of the historic building's demise, saying it captures the "emotion of an entire nation."

"Thinking of all the Catholics and all the French people," Macron wrote. "Like all of our countrymen, I am sad tonight to see part of us burning."

Monday is the start of Holy Week, the busiest and most important period of the liturgical year. Easter is on Sunday.

American Kelly Weymouth, who is currently studying abroad in Paris, said she was taking a sunset cruise on the Seine river with her mother when the fire broke out.

At one point, as the traffic on the river was stopped, Weymouth and about 40 fellow passengers from all over the world watched in horror for about 45 minutes as the fire destroyed the cathedral, she said.

The Frenchmen who were on the boat began to sob and embrace each other, Weymouth said, describing the moment as "very intense."

3:45 p.m. ET:

The Vatican press office issued a statement expressing shock and sadness at the loss of the cathedral, which it described as a "symbol of Christianity in France and in the world."

"We express our closeness to French Catholics and the Parisians and assure them of our prayers for the firefighters and all those that are doing everything possible to fight this dramatic situation," the statement read.

3:50 p.m. ET:

UNESCO released a statement saying it is "standing by France's side to safeguard and restore this invaluable heritage."

4:12 p.m. ET:

The French General Directorate for Civil Security and Crisis Management tweeted that hundreds of firefighters are "doing everything they can" to bring the fire under control. Water-bombing aircraft is not being used to put the fire out because the weight could lead to the collapse of the entire structure, the agency said.

4:40 p.m. ET:

No injuries have been reported, according to fire department officials. Flames have reached one of the towers and brought down the church spire that extended 315 feet high.

"If it collapses, you can imagine how important the damage will be," Gallet said.

4:49 p.m. ET:

All the most precious artwork being stored inside the cathedral has been saved, according to the Parisian fire department.

4:55 p.m. ET:

Paris Mayor Anne Hidalgo said in a news conference that those living near Notre Dame have been evacuated in case the cathedral collapses.

5:07 p.m. ET:

A French firefighters told reporters that the main structure of the cathedral, as well as the two iconic main towers at the front, have been saved.

6:06 p.m. ET:

French President Emmanuel Macron was visibly emotional as he addressed the media outside the Notre Dame Cathedral, describing the blaze as a "terrible tragedy."

Around 500 firefighters were involved in helping to put out the fire, Macron said.

"The worst was avoided even if the battle has not been completely won," Macron said, vowing to rebuild the historic structure by utilizing "the best talents."

ABC News' Ben Gittleson and Kirit Radia contributed to this report.