An estimated 400,000 people have taken to the streets of Paris to protest against same sex couples marrying and adopting children.
President Francois Hollande wants to make homosexual marriage and adoption legal by June.
However demonstrators, backed by the Catholic Church and the right-wing opposition, argue it would undermine an essential building block of society.
The BBC reports people came from all over France to march in the city, many on specially chartered buses and trains.
Three big marches converged on the Champs de Mars, next to the Eiffel Tower.
Organisers, who include a flamboyant comedian known as Frigide Barjot, are determined not to let themselves be typecast as homophobic, so there has been strict vetting of slogans and banners, a BBC correspondent reported.
Centre-right UMP President Jean-Francois Cope said the rally would be a test for the president because millions of French people were probably concerned by the reform.
The far-right National Front is also opposed to the change, although its leader Marine Le Pen stayed away from the march, arguing the issue was a diversion by politicians from France's real problems.