Notre Dame has caught fire, with the flames engulfing the upper part of the cathedral, and the flames licking around its two bell towers.
A major operation was under way, the fire department added, while a city hall spokesman said on Twitter that the area was being cleared.
Fire trucks sped through Paris towards the scene on the Ile de la Cite – an island in the Seine at the heart of Paris.
Officials in Paris said the fire, which started around 7pm local time, could be linked to renovation works being carried out at the cathedral. French media reports, citing police sources, added that the fire started by accident.
Emmanuel Macron, the French president, announced he was cancelling a speech he was due to give this evening due to the conflagration.