The weekend’s actions are meant to put pressure on President François Hollande to keep his campaign promises to legalise gay marriage and open medically assisted reproduction to same-sex couples, Stéphane Corbin of the Fédération LGBT group told RFI.
But activists say they had not planned to hold a national demonstration until opponents succeeded in bringing 100,000 people onto the streets in November, followed by several thousand on 8 December.
They declare themselves shocked by the virulence of the antis, backed by fundamentalist Catholics and the mainstream right-wing UMP party, and fear a revival of homophobia in France.
Many gays were “hurt” by suggestions that “because you are homosexual, because you are homosexual parents, you are challenging society, you are a danger to your children”, inter-LGBT’s Nicolas Gougain told a National Assembly committee earlier this month.
On Saturday demonstrations took place in most of France’s main towns, including Marseille, Lyon, Montepellier.
Families Minister Dominique Bertinotti welcomed them as “a very good thing”.
She claimed that many of the law’s opponents were “the same people who opposed divorce, against contraception, against abortion, against Pacs [civil unions] and they are the same who were against the decriminalisation of homosexuality in 1981”.
Opinion polls show 60 per cent of French people backing gay marriage but only 46 per cent supporting adoption by gay couples.
Several dozen mayors came to Paris on Saturday to demand that the law be scrapped.
Sunday's demonstration is backed by left-wing parties, several trade unions and rights groups.