Parisian firefighters have put their lives on the line to save some of the world's most priceless relics, forming a human chain inside the burning Notre Dame to salvage Jesus Christ's Crown of Thorns.
Mayor Anne Hidalgo thanked the city's first responders in a tweet, revealing they had formed a "fantastic human chain to save the works" of Notre-Dame.
"The Crown of Thorns, the tunic of Saint Louis and several other major works are now in a safe place," Hidalgo wrote in a translated tweet.
Father Fournier, Chaplain of the Paris Firefighters, told reporters he went into the burning cathedral to save the Blessed Sacrament and Crown of Thorns.
The tunic of Saint Louis, another prized — and saved — French relic, has been dated back to the 13th century.
Fears were growing that Jesus Christ's Crown of Thorns could've been destroyed in the devastating Notre-Dame fire after the blaze guttered much of the cathedral's roof and toppled its spire.
A catastrophic fire engulfed the upper reaches of Paris' soaring Notre Dame Cathedral as it was undergoing renovations late Monday night, threatening one of the greatest architectural treasures of the Western world as tourists and Parisians looked on aghast from the streets below.
France's Interior Ministry said firefighters had managed to save the main structure and the building's two bell towers, despite two-thirds of the roof and Notre-Dame's iconic spire destroyed.
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 the Catholic relic, the crown of thorns, which is only occasionally displayed, including on Fridays during Lent.
Authorities feared the stone walls of the church and the amount of oxygen that could feed the fire could cause the temperatures inside the cathedral to be very high, creating a virtual incinerator inside the building.
Cathedral officials report many of Notre-Dame's most important relics have been saved, thanks to the efforts of first responders.
The same cannnot be said for the cathedral's iconic stained glass windows or its detailed roof, nicknamed "The Forest" due to its extensive lattice of woodwork.
The Crown of Thorns has long been considered Paris' equivalent of the Crown Jewels and is stored at the end of a nave in the cathedral.
While the exact age of the relic is unknown, historians have dated it back to Jesus' crucifixion.
The relic originally came from Jerusalem and was carefully placed in the nearby Sainte-Chappelle, a chapel built in the 13th century specifically for the crown.
It is presented to believers for veneration on the first Friday of each month and every Friday during Lent.
The crown was housed in the cathedral's treasury, which also contained other sacred Christian artefacts.
A fragment of the True Cross and one of the Holy Nails used to crucify Jesus Christ were also kept with the Crown of Thorns.
The blaze collapsed the cathedral's spire and spread to one of its landmark rectangular towers. A spokesman said the entire wooden frame of the cathedral would likely come down, and that the vault of the edifice could be threatened too.
"Everything is burning, nothing will remain from the frame," Notre Dame spokesman Andre Finot told French media.
Some 400 firefighters were battling the blaze well into the night. 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 cathedral, watching in shock as acrid smoke rose in plumes.
The 12th-century cathedral is home to incalculable works of art and is one of the world's most famous tourist attractions.
The 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 ($A9.45 million) renovation project on the church's spire and its 250 tons of lead.
Questions are now being asked about if there's anything firefighters could have done to control the blaze sooner?
Experts say the combination of a structure that's more than 850 years old, built with heavy timber construction and soaring open spaces, and lacking sophisticated fire-protection systems left firefighters with devastatingly few options once the flames got out of control.
"Very often when you're confronted with something like this, there's not much you can do," said Glenn Corbett, a professor of fire science at John Jay College.
Fire hoses looked overmatched — more like gardening equipment than firefighting apparatus — as flames raged across the cathedral's wooden roof and burned bright orange for hours.
Some of the factors that made Notre Dame a must-see for visitors to Paris — its age, sweeping size and French Gothic design featuring masonry walls and tree trunk-sized wooden beams — also made it a tinderbox and a difficult place to fight a fire, said US Fire Administrator Keith Bryant.
With a building like that, it's nearly impossible for firefighters to attack a fire from within. Instead, they have to be more defensive "and try to control the fire from the exterior," said Bryant, a former fire chief in Oklahoma and past president of the International Association of Fire Chiefs.
"When a fire gets that well-involved it's very difficult to put enough water on it to cool it to bring it under control," Bryant said.
And while there's a lot of water right next door at the Seine River, getting it to the right place is the problem, he said: "There are just not enough resources in terms of fire apparatus, hoses to get that much water on a fire that's that large."
Because of narrower streets, which make it difficult to manoeuver large ladder fire trucks, European fire departments don't tend to have as large of ladders as they do in the United States, Bryant said.
And what about US President Donald Trump's armchair-firefighter suggestion that tanker jets be used to dump water from above on Notre Dame?
French authorities tweeted that doing so would've done more harm than good. The crush of water on the fire-ravaged landmark could've caused the entire structure to collapse, according to the tweet.
Other landmark houses of worship have taken steps in recent years to reduce the risk of a fire.
St Patrick's Cathedral in New York City, built in 1878, installed a sprinkler-like system during recent renovations and coated its wooden roof with fire retardant. The cathedral also goes through at least four fire inspections a year.
Washington National Cathedral, built in 1912 with steel, brick and limestone construction that put it at less risk of a fast-moving fire, is installing sprinklers as part of a renovation spurred by damage from a 2011 earthquake. That cathedral faces fire inspections every two years.
- additional reporting AP