French President Emmanuel Macron has visited Paris's famous Notre-Dame cathedral, two years to the day after flames ripped through its centuries-old attic and sent its spire crashing through the vaults below.

Macron wore a boiler suit and hard hat to go up on the roof to see for himself the work being done to salvage and restore the landmark.

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In the hours after the blaze, Macron promised a distraught French nation the cathedral, which dates back to the 12th century, would be rebuilt and later said it would be reopened in some form to worshippers by the time of the Olympics in 2024. 

One of the most famous landmarks in a city steeped in monuments burned long into the evening on April 15, 2019 after a likely electrical fault sparked the blaze.

The 850-year-old structure's symbolic spire toppled into the flames as crowds of onlookers stood quietly in shock or cried out as the fire raged. 

Patrick Chauvet, the rector and archpriest of the cathedral, was just outside the building when the fire started. "It was a true ordeal – to see your cathedral burning and not being able to do a thing," he told CGTN Europe. 

"We could only trust in the fire brigade and hope they could save the priceless objects and the famous architecture," he added. 

It took hundreds of firefighters 15 hours to control and finally extinguish the fire. 

Two years later, work is ongoing to secure the site fully so reconstruction work can begin in earnest to try to meet the deadline promised by Macron in 2019. 

From the rooftop on Thursday the president peered down into the cathedral's damaged transept and thanked workers on the site. "We're all impressed with what we see, with the work that has been achieved in two years," Macron told a group of workers with the Paris skyline behind him. "Bravo and thank you."

Donations worth hundreds of millions of dollars have poured in to help pay for the restoration. 

"As things stand, we will need all these donations to complete the necessary work. We need people to keep giving money because it is a work without end," Jean-Louis Georgelin, the former army chief of staff named by Macron to head the renovation, told France Inter radio. 

"We're arriving at a critical juncture." 

Notre-Dame is part of a UNESCO World Heritage Site in central Paris and the United Nations cultural agency sees the renovation plan as an opportunity to show how modern technologies can be used to restore ancient buildings. 

"There were robots used from the very beginning to take out certain materials that fell from the roof into the cathedral when it was not safe to send people inside," said Mechtild Rossler, the director of UNESCO's World Heritage Center. 

"There were drones used to monitor from above the roof's situation and there is 3D reconstruction on computer screens so we can see how it will look like in the future." 

The wood in the old cathedral roof was burnt to ashes and the new beams will come from ancient oaks that have been carefully chosen from every region of France to rebuild the cherished national monument. 

Notre-Dame is a tourist attraction, a cultural beacon and a place of worship, but now it is set to also become a symbol of regeneration.

(With Reuters)