The Church of Scientology and seven of its French leaders went on trial on Monday on charges of organised fraud that could lead to an outright ban on the organisation in France.
Known for its Hollywood celebrity followers Tom Cruise and John Travolta, the Church is in the dock in Paris for the second time in six years, although French courts have prosecuted several individual Scientologists since 1978.
The Paris court was hearing the complaint of a woman who alleges she was manipulated into handing over 20,000 euros (28,000 dollars) for costly Scientology products, such as an "electrometer" to measure mental energy.
The woman said she was approached in a Paris street by a Scientologist in late 1998 who offered a free personality test, at a time when she was feeling psychologically fragile.
After being told that her test results were poor, the woman was sold a series of "life-improvement courses", vitamins and other costly products that landed her into debt.
A second complaint before the court came from a woman who alleged that she was forced by her Scientologist employer to undergo testing and enroll in courses. In the end, she was fired.
French defence lawyers argue that Scientology resorts to harassment and pressure to rein in victims who all show signs of vulnerability.
The Scientology Celebrity Centre in Paris, its director Alain Rosenberg and six other top officials are accused of preying on vulnerable would-be followers "with the goal of seizing their fortune by exerting a psychological hold," they said.
The Church's spokeswoman in France rejected the accusations.
"This is a trial for heresy," said Daniele Gounord, adding that the Church was being "hounded" by French courts.
The seven top Scientologists are also charged with illegal practice of pharmaceutical work after plaintiffs said they were given vitamins and concoctions to improve their mental state.
French prosecutors failed to win a conviction in a previous fraud case against the Church in 2003 but this is the first time that the plaintiffs are invoking a 1994 law that could lead to its dismantling.
Under provisions of that legislation, the Scientology Celebrity Centre and a library in Paris could be shut down for failing to live up to their responsibilities as legal entities.
Founded in the United States in 1954 by science-fiction writer L. Ron Hubbard, the Church of Scientology was officially recognised as a religion there nearly 20 years later.
But it is often accused in European countries of exploiting its members financially. France considers it a sect and not a religion.
The trial is scheduled to continue until June 17.