Notre-Dame cathedral’s spire has collapsed in a “terrible” fire, forcing the French president to cancel a major speech.
Plumes of smoke poured from the historic landmark’s steeples, as flames spread out across the 12th century building.
A city hall spokesman said the area was being cleared, while the fire department added a major operation was under way.
Paris mayor Anne Hidalgo urged the public to “respect the security perimeter” around the cathedral while emergency services tackle the “terrible fire”.
French President Emmanuel Macron also confirmed he had scrapped a planned television speech to the nation.
He said in a message on Twitter: “I am so sad tonight to see this part of all of us burn.”
Footage from the scene shows the cathedral’s spire collapsing.
It was not immediately clear what caused the blaze.
Local media reported that police were treating the incident as an accident.
Home Secretary Sajid Javid called the scenes “heartbreaking”.
The cathedral dates back to the 12th century and is famous for featuring in Victor Hugo’s classic novel the Hunchback of Notre-Dame.
It attracts millions of tourists every year.
Notre-Dame was in the midst of renovations, with some sections under scaffolding, while bronze statues were removed last week for works.