The Notre Dame Cathedral in Paris is to hold its first mass since it was ravaged by fire, as environmental campaigners continued to warn over possible lead pollution and the government confirmed that less than 10% of about €850m pledged by individuals and business leaders for its reconstruction has been paid.

Saturday’s mass, two months after fire destroyed the roof of the 850-year-old landmark, will be held in a side-chapel and limited to about 20 people “for obvious security reasons”, the diocese said.

In attendance will be mainly priests and canons, but not members of the parish. The priests and religious representatives will all have to wear hard hats. Led by the archbishop of Paris, Michel Aupetit, the service will be broadcast live on a religious TV channel.

The cathedral and the square in front of it are closed to the public as a lengthy operation gets under way to make the weakened structure safe. The French culture minister, Franck Riester, said on Friday the cathedral was still “in a fragile state, namely the vault, which hasn’t yet been secured”.

The Paris authorities are continuing to clean the cathedral square and surrounding areas after finding high lead levels on the ground surrounding the cathedral. Environmental campaigners have warned of potential environmental pollution from the hundreds of tonnes of lead in the cathedral spire and roof, which melted in the extreme heat of the fire.

Health authorities have urged children and pregnant women living around the cathedral to have the levels of lead in their blood checked amid concerns about the impact of the fire.

The diocese would like to see the square in front of the cathedral reopened and to begin celebrating evening prayers there. One option could be to build a temporary structure for worship on the square. But authorities have not yet ruled the area safe for the public.

The culture minister confirmed on Friday that less than 10% of around €850m (£757m) very quickly pledged to help restore the cathedral has been paid. Among donors who had swiftly offered funds were the luxury goods billionaires Bernard Arnault and François-Henri Pinault as well as several companies and individuals.

But a Notre Dame press officer said on Friday that so far, workers’ salaries and other costs had mostly been covered by donations from US citizens through the charitable foundation Friends of Notre Dame.

“The big donors haven’t paid. Not a cent,” said Andre Finot. “They want to know what exactly their money is being spent on and if they agree to it before they hand it over, and not just to pay employees’ salaries.”

Riester said: “There could be people who promised to donate then in the end don’t. But more importantly – and this is normal – the donations will be paid as restoration work progresses.”

The French president, Emmanuel Macron, has set a target of five years for restoring the cathedral. His political opponents say it will take longer.