Newly restored Notre Dame cathedral held its first service for the public on Sunday after a historic re-opening ceremony that saw firefighters, builders and artists celebrated for their work saving the 12th-century masterpiece.

The beloved Paris monument nearly burned down in 2019, but has been renovated inside and fitted with a new roof and spire during a frenzied reconstruction effort since then.

During a re-opening service on Saturday, attended by world leaders including incoming US president Donald Trump, French leader Emmanuel Macron expressed the "gratitude of the French nation" for the work.

"We have rediscovered what great nations can do -- achieve the impossible," he said.

One of the most moving moments came when firefighters in their protective gear walked through the congregation to thunderous applause as the word "Merci" ("Thank you") was beamed on the intricate facade and famous belltowers of the Gothic masterpiece.

The architectural wonder had been in danger of collapsing during the April 2019 blaze and was saved only by the heroic intervention of teams of firefighters pumping water from the nearby River Seine.

After the morning service, a second mass in the evening at 6.30pm will be open to the public, with roughly 2,500 people who secured free tickets this week expected to attend.

The cathedral will open fully to visitors on December 16 via an online reservation system.

The exact cause of the 2019 blaze has never been identified despite a forensic investigation by prosecutors, who believe an accident such as an electrical fault was the most likely reason.

Notre Dame welcomed around 12 million visitors a year before the fire, but expects to receive an even higher figure of "14 to 15 million" after the reopening, according to the church authorities. (AP)