Notre Dame Cathedral is a 850-year-old gothic masterpiece that had been undergoing €6 million restoration work.
The entire wooden interior of the 12th-century landmark was burning and was likely to be destroyed.
Emergency services were trying to salvage the artwork and other priceless items stored there, the city's deputy mayor, Emmanuel Grégoire, said.
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 'Holy Crown' - said to be the wreath of thorns placed on Jesus' head before he was crucified.
It's not known whether the relics were saved before the fire took hold.
Notre Dame is one of the finest buildings of the medieval era still standing.
Visitors gaze at its bells, ornate flying buttresses, spectacular stained-glass windows and visit its vast choir and nave.
The choir has carved wooden stalls and statues representing the Passion of the Christ. The organ has 8000 pipes.
Centuries-old sculptures and paintings line the walls.
There are three spectacular rose windows, treasury and bell towers.
Gargoyles look over Paris from the top of the western façade reached from the North Tower.
The treasury contains artwork and relics including the 'Holy Crown' said to be the wreath of thorns placed on Jesus' head before he was crucified.
The paintings include the Mays of Notre-Dame - large paintings commissioned almost every year, from 1630 to 1707, by the Parisian Goldsmiths' Society in honour of the Virgin Mary. The Visitation by Jean Jouvenet.
Built in the 12th and 13th centuries, Notre Dame is the most famous of the Gothic cathedrals of the Middle Ages as well as one of the most beloved structures in the world.
Situated on the Ile de la Cite, an island in the Seine river.