Notre Dame Cathedral is a 850-year-old gothic masterpiece that had been undergoing €6 million restoration work.
The entire wooden interior of the 12th-century landmark was burning and was likely to be destroyed.
Emergency services were trying to salvage the artwork and other priceless items stored there, the city's deputy mayor, Emmanuel Grégoire, said.
Among the most celebrated artworks inside are its three stained-glass rose windows, placed high up on the west, north and south faces of the cathedral.
Its priceless treasures also include a Catholic relic, the 'Holy Crown' - said to be the wreath of thorns placed on Jesus' head before he was crucified.