The French Parliament on Saturday adopted the main clause of a bill that would allow same-sex marriage and grant gay couples the right to adopt children.
Deputies voted 249-97 to back the clause eliminating opposite gender as a condition of the right to marriage.
The draft law, the first major social reform of Francois Hollande's Presidency, has sparked major protests.
Several hundred thousand people massed at the Eiffel Tower in Paris last month to protest against the plan.
The approval of the key clause prompted fresh protests in several towns on Saturday.
In Paris, around a thousand people holding placards saying "We are all born of a man and a woman" gathered near the Invalides monument, not far from the parliament building, a few hours after the deputies' vote.
Protesters in the south-eastern town of Lyon assembled outside the town hall to voice their opposition to same-sex marriage but also medically assisted procreation and surrogate motherhood, which are not included in the draft law.
More than 5,000 amendments have been presented for the bill, which deputies began debating on Tuesday.
The debate is expected to last two weeks.
Socialist deputies had originally planned to amend the marriage bill to include access to assisted reproduction techniques for lesbians, but pulled back when this idea appeared especially controversial and threatened to hinder the wider marriage reform.
However, opinion polls have shown the number of people supporting the legalisation of same-sex marriage in France has risen despite the protests.
The proportion of those supporting the change in the law rose to 63% in late January from 60% in early January and December, according to a poll.
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