It's unclear what caused the fire but officials are treating it as accidental, Reuters reported.
Police have said the blaze is thought to be linked to the structure's ongoing $6.8 million renovation project.
It is not possible to think of Paris without thinking of Notre-Dame.
"The horrific fire that is engulfing the Cathedral of Notre-Dame de Paris is shocking and saddens us all, for this particular cathedral is not only a majestic Church, it is also a world treasure".
The fire came less than a week before Easter amid Holy Week commemorations.
French President Emmanuel Macron is on his way to the scene.
"From our hotel room we watched the big tall spire go straight down into the nave of the church", Debbie Furr said. He pledged to rebuild the church and said he would seek global help to do so.More news: Disney+ to launch in November, priced at $6.99 monthly
"Everything is burning, nothing will remain from the frame", Notre Dame spokesman Andre Finot told French media, according to the AP. Its bells rang out calling all Parisians to celebrate the end of the Second.
Paris Fire brigade members are seen at an entrance that looks into the cathedral.
The smoke-eater was one of roughly 400 firemen to be called down to the iconic Parisian cathedral on Monday after it went up in flames.
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.
French historian Camille Pascal told BFM broadcast channel the blaze marked "the destruction of invaluable heritage".
A centuries-old crown of thorns made from reeds and gold and the tunic worn by Saint Louis, a 13th century king of France, were saved, Notre-Dame's top administrative cleric, Monsignor Patrick Chauvet said.
While experts said that the cathedral was not at risk of sudden collapse, many will now be fearing the worst, as parts have already collapsed.
He added: "We can be only horrified by what we see".