Dramatic pictures and videos shared on social media show smoke and flames rising from the landmark in France’s capital as huge crowds gathered.
Firefighters confirmed a blaze had started at the medieval cathedral.
They said that a major operation is under way and a city hall spokesman added that the area was being cleared.
The cause of the fire at the 850-year-old Gothic building is not yet known.
Officials said it could be linked to renovation works being carried out on the cathedral.
More to follow.