The entire spire of Notre Dame's historic cathedral in Paris on fire

Notre Dame Cathedral-- one of the most popular landmarks in Paris -- is going up in flames.

Shocked and horrified tourists are posting photos and videos that show flames in and around the building's huge spire. Smoke is billowing up and filling the sky over Paris.

Firefighters are racing to the scene, and according to one reporter, the fire might have been sparked by restoration work that's being done at Notre Dame.