Some social media users turned to Twitter to share images and videos showing smoke coming out of the cathedral.
"Notre Dame Fire in progress", police said on Twitter.
Smoke could be seen from a distance billowing out from the top of the medieval cathedral, while flames leaped out from two of its bell towers.
The President of the United States, Donald Trump, has also tweeted his concern.
French TV was reporting that the fire had spread to the giant spire of the building.
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Loughlin and Giannulli are now facing charges of conspiracy to commit fraud and conspiracy to commit money laundering. So that is something that has understandably made her less likely to enter a plea, ' the unnamed person told People .
Authorities said the fire could be linked to renovation work.
Mayor of Paris Anne Hidalgo labelled the fire "terrible" despite the extent of the damage being unclear at this stage.
The sight stopped pedestrians in their tracks along the Seine River, which passes under the cathedral.
The cathedral, which was built in 1160, is engulfed in flames and firefighters are struggling to contain the massive fire.
Macron's pre-recorded speech was set to be aired later Monday on French TV. "I ask everyone to respect the security perimeter". An area surrounding the building in central Paris has been cleared, officials said.