Hundreds of thousands of demonstrators poured into Paris to protest against gay marriage laws that will come into force in June under plans drawn up by the French president, Francois Hollande.
Police expect about 300,000 people to march on the Eiffel Tower from three points in the city, in what could be the largest demonstration in a decade. Various counter-demonstrations were also planned, Le Monde reported.
A massive police presence, bolstered by military and security forces, is to be deployed on the streets of the French capital.
The marriage proposals would grant same-sex couples the right to adopt children, along with other legal entitlements. Polls show about 50 percent of French voters support gay marriage, down from 65 percent in August, while less than half now support the granting of adoption rights to homosexuals.
But while the plans have proved nationally divisive, they have rallied the opposition, uniting disparate groups including Catholics, Muslims, Jews, far-rightists and even homosexuals in increasingly vocal opposition.
A coalition of the Catholic establishment, traditional families, conservatives and evangelicals has been mobilised to oppose the moves.
The demonstrations caused major disruptions to transport and forced the closure of metro stations as protesters streamed into Paris by bus, car and specially reserved high-speed train.
"We want this draft law to be withdrawn," protest organiser Patricia Soullier told BFM-TV as she boarded a Paris-bound train in the southern city of Marseille.
The proposals formed part of Hollande's election manifesto, and are likely to be passed without difficulty by parliament. The president angered many with what was seen as a clumsy attempt to avoid public debate and slip the reform through parliament.
Cardinal Andre Vingt-Trois, head of the Catholic Church in France, rallied the opposition with a sermon in August. Muslim, Jewish, Protestant and Orthodox Christians joined him in speaking out.
The problems that children could face in same-sex households became a key issue, uniting the various groups opposed to the measure.
Plans to allow lesbian couples access to artificial insemination, which is currently limited to heterosexual couples with fertility problems, have been dropped in face of the mounting pressure.
Organisers insist they are protesting in favour of traditional marriages, rather than against gays and lesbians. "We are marriagophile, not homophobe," Frigide Barjot, a protest organiser and author of a book entitled Confessions of a Trendy Catholic, told France24.
"I'm happy many Catholics will be mobilised, but this is not a church demonstration against the government," said Cardinal Vingt-Trois, who says he will go to meet the marchers, but will not join them.
Though most national faith leaders will not join the protest, at least eight Catholic bishops have confirmed they will be marching.
Opposition leader Jean-Francois Cope and other conservatives, as well as leaders from the far-right National Front, will march as private citizens without political banners, Reuters reported.
Civitas, a far-right Catholic group whose protests have been openly homophobic, plans a rival march that will run parallel to the main columns.