Its cause is not yet clear, but officials say it may be linked to renovation work.
The peak of the church is undergoing a 6-million-euro ($9.1-million) renovation project.
It's unclear if anyone has been hurt in the fire.
"What we see from the outside would suggest catastrophic damage on the inside", CNN's Melissa Bell reported.
Portland woman studying in NY dies after falling from clock tower
However, a senior, Anne, who declined to give her last name, said that "the doors aren't always locked". Fordham Public Safety is still investigating how Monfries and her friends were able to get inside.
Notre Dame is distinguished for its size, antiquity and architectural interest.
The fire broke out in the cathedral on Monday afternoon, a Paris fire department confirmed, adding that a major operation to tackle the fire is underway.
The cathedral dates back to the 12th century and is famous for its part in Victor Hugo's novel the Hunchback of Notre-Dame.
The police urged people to avoid the area and facilitate the passage of emergency vehicles. Perhaps flying water tankers could be used to put it out. Massive plumes of yellow brown smoke is filling the air above Notre Dame Cathedral and ash is falling on tourists and others around the island that marks the center of Paris.