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 "formidable 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," Ms 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 cathderal to be very high, creating a virtual incerator 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.