Several hundred thousands of protesters massed at the Eiffel Tower in Paris on Sunday to oppose a plan by President François Hollande to legalize same-sex marriage and adopt by June.
The organizers put the number of protesters at 800,000, while police estimated 340,000 people had gathered.
Although France allows civil unions between same-sex couples, Hollande made a pledge to extend their rights as part of his presidential campaign.
“The French are tolerant, but they are deeply attached to the family and the defense of children,” said Daniel Liechti, vice president of the National Council of French Evangelicals, which urged its members to join the march.
Organizers said that they are not against gay and lesbian people but for the rights of children to have a father and mother.
Support for gay marriage in France has slipped by about 10 percentage points to below 55% since opponents began speaking out, according to surveys, and fewer than half of those polled recently wanted gay people to win adoption rights.
Opponents of gay marriage and adoption, including most religious leaders in France, have argued that the reform would create psychological and social problems for children, which they believe should trump the desire for equal rights for gay couples.
Same-sex weddings are legal in 11 countries including Argentina, Belgium, Canada, Denmark, Iceland, Netherlands, Norway, Portugal, Spain, South Africa, Sweden, as well as nine American states and the District of Columbia.