The April 15 blaze caused the roof and spire of the architectural masterpiece Notre-Dame to collapse.

A small congregation wearing hard hats will attend mass at Notre-Dame Cathedral in Paris on Saturday, the first service to be held since fire devastated the Gothic landmark two months ago.

Church leaders are keen to show life goes on at the cathedral as donations to help rebuild it trickle in. Less than 10% of the 850 million euros pledged by billionaires, business leaders and others has been received so far, the French government said.

Saturday's mass, which commemorates the cathedral's consecration as a place of worship, is due to be held at 1600 GMT in a side-chapel, with attendance limited to about 30 people who will wear the protective headgear for safety reasons.

"It is a nice symbol. A very small group of people will attend and one can understand why as there are still major safety issues," Culture Minister Franck Riester told Europe 1 radio.

He told France 2 television on Friday the cathedral was still "in a fragile state, namely the vault, which has not yet been secured. It can still collapse".

The April 15 blaze caused the roof and spire of the architectural masterpiece to collapse, triggering a worldwide outpouring of sadness as well as the multi-million-euro pledges for reconstruction work.

Among the high-profile people who promised to donate to the rebuilding effort were luxury goods tycoons Bernard Arnault and Francois-Henri Pinault.

"There could be people who promised to donate then in the end did not," Riester said, without giving further details. "But more importantly, and this is normal, the donations will be paid as restoration work progresses."

French President Emmanuel Macron has set a target of five years for restoring the cathedral, though Riester was more cautious.

"The president was right to give a target, an ambition. But obviously what matters in the end is the quality of the work," he said. "So it does not mean that work will be totally finished in exactly five years."

The archbishop of Paris, Michel Aupetit will lead Saturday's service, which will be broadcast live on a religious TV channel.