"When the tower, that central tower fell, it was a moan and a gasp from the crowd in Paris, exactly as there was in St. Boniface when the bell towers collapsed", Mailhot said Tuesday morning.
View of the burning Notre Dame from along the River Seine.
While the scale of the damage remains unclear, donations to assist in efforts to rebuild poured in overnight.
A plan to safeguard the masterpieces and relics was quickly put into action after the fire broke out.
Some of the works were being transferred from City Hall to the Louvre, where they will be dehumidified, protected and eventually restored.
They can be seen fearlessly dashing inside which is being ravaged by fire.
Paris Deputy Mayor Emmanuel Gregoire described authorities' "enormous relief" at the salvaging of pieces such as the Crown, which were transported to a "secret location" after the fire.
At dawn, the twin 69-metre towers swarmed with building specialists and architects. He added that workers were "surveying the movement" of the structure and the contents in it. A spokesman for the cathedral told AFP that the wooden structure supporting the roof was being gutted by the blaze.
Investigators interviewed witnesses overnight and began speaking with employees of five different construction companies that were working on the monument, said public prosecutor Remy Heitz.
Crowds of tourists and Parisians watched in horror as the roof of the cathedral came crashing down to the ground. The second alert was sounded at 1643 GMT (6:43 pm) and a blaze was discovered in the roofing at that point. But "it all depends on human beings", he said.
News that the fire was probably accidental has done nothing to ease the national mourning. It was hit when the spire collapsed, said Laurent Prades, heritage director for Notre Dame.
The Archbishop of Paris Michel Aupetit told RMC radio, "Notre-Dame was destroyed but the soul of France was not".
The tragedy prompted an outpouring of support internationally, with the Queen saying she was "deeply saddened" and Pope Francis offering his prayers. His son, François-Henri Pinault, 56, who heads the family firm, said: "It's the nation".
French officials said they do not suspect foul play.
According to one tally, the amount pledged crossed $700 million.
Sister Madeleine, a Dominican nun from Paris, was visibly upset when she arrived to inspect Notre Dame, a day after the blaze.
There is no indication that the fire was linked to terror but ISIS fanatics have revelled in the damage to the 850-year-old landmark, a symbol of Western civilisation, just days before Easter.
"The organ is a very fragile instrument, especially its pipes".
A tourist is looking for what she believes to be a dad and daughter whom she photographed sharing a heart-warming moment outside the Notre Dame Cathedral in Paris just before the historic landmark caught fire. Macron is holding a special Cabinet meeting Wednesday dedicated to the Notre Dame disaster. He called the structure "a part of us", referring to the country of France, and he asked for help repairing it.