Police and fire services will spend the next 48 hours assessing the “security and safety” of Notre Dame Cathedral after a fire at the historic monument in the centre of Paris on Monday evening.

Laurent Nunez, a junior interior minister, said on Tuesday that the primary concern of French authorities was identifying weaknesses in the centuries-old structure.

“We have identified some vulnerabilities in the structure … notably in the vault and the north transept pinion that needs securing,” Nunez said. He said this work would take 48 hours and residents of five buildings around the north transept were being evacuated.

Architects have identified three main holes in the structure, in the locations of the spire, the transept and the vault of the north transept. Most of the wooden roof beams have been burned, and parts of the stone vaulting holding up the roof have collapsed.

As well as damage from the heat, which firefighters said reached more than 800C, experts also need to assess damage from the vast quantities of water that the firefighters poured into the cathedral.

French interior minister, Christophe Castaner, said the cathedral was “under permanent surveillance because it could still move.”

The fire was discovered at 6.43pm local time at the base of the spire, which was constructed of wood and lead in the 19th century, and reached a height of 93 metres from the ground. The flames quickly spread to the cathedral’s ribbed roof, made up of hundreds of oak beams, some dating back to the 13th century, and known as “la forêt” (the forest).

Nunez praised the actions of the Paris fire service, who he said had battled “not just outside but inside the building at great risk to their own lives”.

The 500 firefighters at the scene had battled to prevent the flames reaching the two belfry towers, where the cathedral bells hang. If the wooden frame of the towers had caught fire it could have sent the bells – the largest of which, the Emmanuel bell, weighs 13 tonnes – crashing down, potentially causing the collapse of both gothic towers. “They saved the edifice but it all came down to 15-30 minutes,” Nunez said.

Gabriel Plus, a spokesman for the Paris fire service, said: “We are satisfied and grateful that in risking their lives [firefighters] safeguarded the structure of the two belfries, the towers – and the works of art. Now I can confirm the fire is completely out.”

The culture minister, Franck Riester, said religious relics saved from the cathedral, including the Crown of Thorns and Saint Louis’s tunic, were being securely held at the Hôtel de Ville, and works of art that sustained smoke damage were being taken to the Louvre where they would be dried out, restored and stored.

He said three stained-glass “rose” windows did not appear to be damaged but would be examined more closely when the cathedral was made safe. Photos from inside the monument suggest Notre Dame’s grand organ, built in the 1730s and boasting 8,000 pipes, was spared from the flames.

Sixteen copper statues that decorated the spire, representing the 12 apostles and four evangelists, had been removed for restoration only a few days before the fire. Relics at the top of the spire are believed lost as the spire was destroyed.

As well as damage from the heat, which firefighters said reached more than 800C, experts also need to assess damage from the vast quantities of water that the firefighters poured into the cathedral.

Police have begun questioning workers who were carrying out renovations at the cathedral. The Paris prosecutor’s office has opened an inquiry into “involuntary destruction by fire”, indicating they believe the cause of the blaze was accidental rather than criminal.

Julien Le Bras, from the scaffolding company involved in the renovation, said his company had worked on a number of historic buildings in France, including the Louvre, and none of his workers were at the site when the fire broke out.

“It is with unqualified sadness I am speaking,” he told a press conference. “The police investigation is taking place and our workers will help answer questions with no reserve whatsoever to throw light on the cause of this drama. At the moment the fire started, none of the workers of my company was present at the site. All security and fire requirements were respected.”

Monseigneur Michel Aupetit, the archbishop of Paris, said people around the world loved “this extraordinary cathedral”. “When we arrived yesterday evening we wanted to cry and people were crying around us,” he said. “Notre Dame is a symbol; more than a symbol, it is the soul of France.”

He told BFMTV: “Notre Dame de Paris on the Île de la Cité , [in the] centre of Paris, is charged with history, with events through the centuries. It draws people, they come to look and they respect the place and they come to pray.”

Emmanuel Macron, the French president, who was at the scene on Monday evening, said a fundraising campaign would be launched immediately, as well as an appeal to international experts to help rebuild Notre Dame.

Two of France’s richest tycoons pledged millions to the fund. It was announced that €100m had been pledged from the family fortune of François Pinault, the founder of Kering, which owns fashion labels including Alexander McQueen and Gucci. Shortly afterwards, Bernard Arnault, the chief executive of the LVMH group, announced a donation of €200m.

French luxury and cosmetics group L’Oreal, along with the Bettencourt Meyers family and the Bettencourt Schueller foundation, also said they would donate €200m. The energy company Total is to give €100m.

The mayor of Paris, Anne Hidalgo, called for a “donors’ conference”, not only to raise money for the reconstruction but to involve experts from around the world.

The UN’s Paris-based cultural agency, Unesco, promised to stand “at France’s side” to restore the site, which it declared a world heritage site in 1991.

“We are already in contact with experts and ready to dispatch an urgent mission to evaluate the damage, save what can be saved and start elaborating measures for the short and medium term,” Unesco’s secretary general, Audrey Azoulay, said on Tuesday.

• This article was amended on 18 April 2019 to improve the translation of Christophe Castaner’s comments and to correct the following architectural details: the vaulting is stone, not concrete; the spire reached a height of 93 metres from the ground, not from the roof; and the beams were not 110 metres long.