The fire may potentially involve renovation works being carried out at the site, the fire service said.
CNN reports In a tweet, police said: "Notre Dame Fire in progress".
Numerous photos and videos uploaded by social media users show clouds of smoke coming out of the building.
Construction on Notre-Dame Cathedral began in 1163 and concluded in 1345.
Local media reports as well as Reuters stated that tourists and locals in the vicinity were taken to safety nearly immediately after the fire was first noticed.
French President Emmanuel Macron has postponed a televised speech to the nation because of the fire. We are mobilized on the spot in close connection with the @dioceseParis.
Smoke and fire is seen leaping from the top of Notre Dame, the iconic Paris cathedral.
Paris Mayor Anne Hidalgo is in despair at the "terrible fire".
The cathedral, which dates back to the 12th century, features in Victor Hugo's classic novel "The Hunchback of Notre-Dame".
In 2012, the Archbishop of Paris and the Diocese of Paris founded "The Friends of Notre Dame" to raise funds for the structural deterioration, with renovation and fix efforts ongoing.