Doctors, interns, nurses and dentists have been protesting for weeks, with some staging a two-week stoppage before Christmas, to amend a controversial health bill that goes before a parliamentary committee from Tuesday.
An interns' association claimed 50,000 people took part in the rally while a surgeons' group put the numbers at 46,000.
The draft legislation notably provides for doctors to be paid in full by the state-run social security system by 2017, rather than the current system under which patients pay doctors and are then refunded by the system.
Medics fear the proposed change will hugely add to their paper-work, cause payment delays and open the door to administrative meddling in medical affairs.
(The protesters reach the Dome of the Invalides. Photo: AFP)
"It's like Thatcher's Britain," said the unions.
Dr Eric Henry who led Sunday’s march told The Local that France’s health service will no longer be the envy of the world if the reforms pass.
“This is a battle to save the French model. When we look at other countries we are always proud to be French because we know at the end of the day we will receive good health care here,” said Henry.
"This is the country where everyone around the world would like to receive treatment."