Some 28 million euros ($30 million) were allegedly stolen in the case, dubbed "red card" because of the dozen football clubs among the plaintiffs.

Some 850 of the victims are represented at the trial which had to be moved to the conference centre in the eastern French city of Nancy because there were too many for a traditional court. Hearings are set to last four weeks.

The defendants, between 28 and 73, are accused of running websites offering fraudulent investment opportunities in diamonds or cryptocurrencies between 2016 and 2018.

One person handed over 400,000 euros believing they were investing in diamonds and what was advertised as a "diamond savings plan".

Some investors lost big chunks of their savings or even contracted loans to invest, attracted by a promise of large annual returns, said Colman, a law firm representing around 100 plaintiffs.

"We believe this trial marks a strong signal in the fight against international financial fraud," the firm said in a statement.

The accused opened 199 bank accounts in 19 countries to receive and transfer funds, according to investigators. Some 2.8 million euros were recovered and could be used to compensate victims.

A side business for the alleged fraudsters was swindling money out of French football clubs.

Individuals based in the southern city of Marseille and in Israel pretended to be agents acting on behalf of professional players. They said the players had changed their banking details and salaries should be sent to the new accounts.

Around 10 top clubs were targeted, with Sochaux, Angers and Toulouse falling for the scam and handing over around 60,000 euros.

The 22 defendants -- including three who are still at large and are being tried in absentia -- have varying degrees of implication in the fraud, prosecutors said.

About 12 are charged with "criminal conspiracy", notably for allowing their names to be used to open bank accounts through which funds were channelled across Europe and to Israel.

Others are accused of "fraud committed in a gang", most of them for building fake websites or ordering fake diamonds.