Jean-Marc Sauve, head of a commission set up by the Catholic church, said a previous estimate last June of 3,000 victims was “certainly an underestimate”.
“It’s possible that the figure is at least 10,000,” he said.
But victims’ groups said they suspected even that was an underestimate, AFP reported.
“It’s the tip of the iceberg. It’s certainly even more,” said Veronique Garnier from the group Faith and Resilience.
Jean-Pierre Sautreau, who heads a victims’ group in the Vendee region in western France said: “10,000 is a lot but it’s the bottom of the scale. We’re far from the truth.”
A hotline set up in June 2019 for victims and witnesses to report abuse received 6,500 calls in its first 17 months of operation.
“The big question for us is ‘how many victims came forward?’ Is it 25 per cent? 10 per cent, 5 per cent or less?” Mr Sauve told reporters.
The Bishops’ Conference of France agreed in November 2018 to set up the commission after huge and repeated child abuse scandals shook the Catholic church at home and abroad.
Victims’ groups welcomed attempts to encourage survivors to speak out, but questioned prosecutors’ willingness and ability to press charges.
The commission, comprised of more than 20 figures with legal, academic and medical backgrounds, was originally scheduled to deliver a final report by the end of last year but has postponed it to September.
Allegations against priests and senior Catholic figures have led to payouts and prosecutions worldwide, as well as changes to church doctrine.
In May 2019, Pope Francis passed a landmark new measure obliging anyone in the church who knew about sex abuse to report it to their superiors.