André Finot, a spokesman for the cathedral, said in a telephone interview that the spire of the cathedral had caught fire. The cause of the fire remained unknown, and there was no immediate indication that anyone had been hurt.
The fire alarm went off around 6:30 p.m., Mr. Finot said, adding that the cathedral had been evacuated.
Video filmed by onlookers and shared on social media showed smoke and flames billowing from the top of the cathedral, engulfing scaffolding and the spire.
Mayor Anne Hidalgo of Paris said on Twitter that “a terrible fire” had struck at the cathedral.
“The Paris firefighters are currently trying to stop the flames,” Ms. Hidalgo said. “I ask everyone to respect the security boundaries.”