Thousands of people, including actress Kirstie Alley and singer Isaac Hayes, went to Paris to protest at the French government's attitude toward the Los Angeles based Church of Scientology.
The protesters, from all over the world, gathered in the city centre but organisers say they were not given official clearance to march.
The Church of Scientology has a contentious relationship with France and figures on a list of 178 groups to be tracked to prevent cult activities.
The Church is seeking recognition as a legitimate religion in Europe. "I represent scientology and other minority religions - which aren't really such a minority," Alley said. "Our freedom of thought is under siege. We have to ensure the legal, moral religious rights of this great country that is invaluable to the artists of the world," she said.
Hayes also said he came to express his support and solidarity with those being oppressed.
"If it's religion today, what will it be tomorrow?" he said. "I say to France: Practise what you preach and live up to your constitution. You've been there and you know what it's like. Be honest. Be real."
Many Europeans have been sceptical of Scientology, whose prominent members include Tom Cruise and John Travolta, as well as Alley.
Founded in 1954 by science-fiction writer L Ron Hubbard, the organisation teaches that technology can expand the mind and help solve problems.