From the New York Times:
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. The cathedral, one of Paris’s most famous landmarks, is currently undergoing renovation work, and several statues were lifted by crane from the spire last week.
“A terrible fire is taking place at the Cathedral of Notre-Dame,” Mayor Anne Hidalgo of Paris said in a tweet, adding that firefighters were trying to bring the flames under control. “I ask everyone to respect the security boundaries.”