Hundreds of thousands of people have converged on the Eiffel Tower to protest against the French president's plan to legalise gay marriage and thus allow same-sex couples to adopt and conceive children.
The opposition to president Francois Hollande's plan has underscored divisions among the secular-but-Catholic French, especially between more traditional rural areas and urban enclaves. But while polls show the majority of French still support legalising gay marriage, that backing gets more lukewarm when the issue of conceiving and raising children comes into play.
The protest march started at three points across Paris, filling boulevards throughout the city as demonstrators walked three miles to the grounds of France's most recognisable monument.
Paris police estimated the crowd at 340,000, making it one of the largest demonstrations in Paris since an education protest in 1984.
"This law is going to lead to a change of civilisation that we don't want," said Philippe Javaloyes, a literature teacher who arrived with 300 people from Franche Comte in the far east. "We have nothing against different ways of living, but we think that a child must grow up with a mother and a father."
Public opposition spearheaded by religious leaders has chipped away at the popularity of Mr Hollande's plan in recent months. About 52% of French favour legalising gay marriage, according to a survey released on Sunday, down from as high as 65% in August.
French civil unions, allowed since 1999, are at least as popular among heterosexuals as among gay and lesbian couples. But that law has no provisions for adoption or assisted reproduction, which are at the heart of the latest debate.
Mr Hollande's Socialist Party has sidestepped the debate on assisted reproduction, promising to examine it in March after party members split on including it in the latest proposal. That has not assuaged the concerns of many at Sunday's protest, however, who fear it is only a matter of time.
Harlem Desir, the leader of Mr Hollande's Socialist Party, said the protest would not affect the proposal's progress. The Socialists control Parliament, and a vote is expected by the end of January.
"The right to protest is protected in our country, but the Socialists are determined to give the legal right to marry and adopt to all those who love each other," he said. "This is the first time in decades in our country that the right and the extreme right are coming into the streets together to deny new rights to the French."