The fire broke out at around 1700 GMT local time, according to French media. It may be linked to renovation works being carried out at the medieval cathedral, one of the capital's tourist magnets.
As it burned into the evening, firefighters battled to prevent one of the main bell towers from collapsing and tried to rescue religious relics and priceless artwork. One firefighter was seriously injured - the only reported casualty.
"The worst has been avoided, even if the battle has not been totally won yet," French President Emmanuel Macron told reporters at the scene shortly before midnight, as firefighters worked to further cool some of the interior structures still at risk of collapse.
Macron said France would launch a campaign to rebuild the cathedral, including through fundraising efforts and by appealing to "talents" from overseas to contribute.
"We will rebuild it together. It will undoubtedly be part of French destiny and our project for the years to come," a visibly moved Macron said.
Paris fire chief Jean-Claude Gallet said the cathedral's main structure had now been saved from complete destruction.
Distraught Parisians and stunned tourists gazed in disbelief as the inferno raged at the cathedral, which sits on the Ile de la Cite, an island in the River Seine and marks the very centre of Paris.
Thousands of onlookers lined bridges over the Seine and along its embankments, held at a distance by a police cordon. Some sang Ave Maria, with others in the crowd also kneeling and praying.
World leaders expressed shock and sent condolences to the French people.
A huge plume of smoke wafted across the city and ash fell over a large area. People watching gasped as the spire folded over onto itself and fell into the inferno.
Firefighters battled smoke and falling drops of molten lead as they tried to rescue some of Notre-Dame's treasures.
A centuries-old crown of thorns made from reeds and gold and the tunic worn by Saint Louis, a 13th century king of France, were saved, Notre-Dame's top administrative cleric, Monsignor Patrick Chauvet said. But firefighters had struggled to take down some of the large paintings in time, he said.
The Paris prosecutor's office said it had launched an inquiry into the fire. Several police sources said that they were working on the assumption for now that it was accidental.
