The French National Assembly has approved the most important article of a bill to legalise same-sex marriage.
Deputies voted 249-97 in favour of redefining marriage as being an agreement between two people - not just between a man and a woman.
President Francois Hollande's Socialists and their left-wing supporters backed it, opposed by the opposition UMP and many centrist MPs.
The proposals have generated protests and counter-protests for months.
Opinion polls suggest that around 55-60% of French people support gay marriage, though only about 50% approve of gay adoption.
The draft bill, which also includes provisions to allow gay couples to adopt children, marks one of France's biggest social reforms since the abolition of the death penalty in 1981.
It is expected that the legislation will reach the statue books by the middle of the year, AFP reports.
In September last year, Cardinal Philippe Barbarin, the Roman Catholic archbishop of Lyon, argued that plans to redefine the concept of marriage would open the door to incest and polygamy.
The debate in the National Assembly is due to last throughout the weekend.