The verdict is "an Inquisition of modern times," said Scientology spokeswoman Agnes Bron, referring to efforts to rout out heretics of the Roman Catholic Church in centuries past.
The head of an association that helps victims of sects, Catherine Picard, called the verdict "intelligent."
"Scientology can no longer hide behind freedom of conscience," she said.
The Los Angeles-based Church of Scientology, founded in 1954 by the late science fiction writer L. Ron Hubbard, has been active for decades in Europe, but has struggled to gain status as a religion. It is considered a sect in France and has faced prosecution and difficulties in registering its activities in many countries.
Defense lawyer Patrick Maisonneuve said during the trial that neither the Church of Scientology nor the six leaders on trial had gained financially from the group's practices.
The original complaint in the case dates back more than a decade, when a young woman said she took out loans and spent the equivalent of euro21,000 on books, courses and "purification packages" after being recruited in 1998. When she sought reimbursement and to leave the group, its leadership refused. She was among three eventual plaintiffs.
Olivier Morice, lawyer for civil parties in the case, said the verdict was "historic" because it was the first time in France that the Church of Scientology has been convicted of organized fraud.
Investigating judge Jean-Christophe Hullin spent years examining the group's activities, and in his indictment criticized what he called the Scientologists' "obsession" with financial gain and practices he said were aimed at plunging members into a "state of subjection."