Thousands of French surgeons began an open-ended strike on Monday to protest against what they consider restrictions to their freedom and pay.
The protest has been called by 30 unions and professional bodies who are making various demands, with organisers claiming a significant impact in some regions.
Hospital surgeons' union Bloc is leading a protest against an agreement accepted by the three largest doctors' unions at the end of October, which says any fees over 2.5 times the basic state tariffs is excessive.
The surgeons argue they have the right to charge higher rates.
Meanwhile, state hospital interns, who have joined in the strike, are fighting against feared new moves limiting the freedom of young doctors to work where they want.
Thousands of interns wearing white coats and surgical masks marched in Paris shouting: "Tomorrow's health lies with young doctors" and "No to discounted medical care".
Health Minister Marisol Touraine on Monday insisted that the October agreement was a "good" one.
Bloc official Xavier Gouyou-Beauchamp said there was a very strong response to the call, adding that 70 percent of private clinics would be affected.
In Strasbourg, the protest made a huge impact with 95 percent of surgeons on strike, said Guillaume Lohr, the head of three private clinics in the eastern city.
The national surgeons' union meanwhile said up to 70 percent of the operating theatres had been affected in Brittany.
Paris public hospitals body AP-HP said a clearer picture of the magnitude of the walkout would only be known later. It said some operations had been cancelled but emergencies would be dealt with by other surgeons who had been requisitioned.
Some politicians and consumer bodies have voiced concern that many doctors -- especially specialists -- are setting very high fees.
Other issues raised by some of the strikers include legal changes that could make patients use doctors selected by the top-up insurers to be better reimbursed.