France’s Interior Ministry said firefighters are now optimistic they can save the main structure and the building’s two bell towers, despite two-thirds of the roof and Notre-Dame’s iconic steeple 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 a Catholic relic, the crown of thorns, which is only occasionally displayed, including on Fridays during Lent.
Fears are growing the relic could’ve been destroyed in the fire.
Authorities fear 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.
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 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.