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 many opposition UMP and 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.
Correspondents say the ease with which the article passed suggests the bill as a whole will pass.
Debates are expected to go on for more than a week, as MPs discuss hundreds of amendments, most of them filed by the centre-right opposition.
On the way they are expected to approve the other key measure in the bill, which would allow gay couples to adopt children.
'Freedom to choose'
The bill marks one of France's biggest social reforms since the abolition of the death penalty in 1981.
"We are happy and proud to have taken this first step," Justice Minister Christiane Taubira said.
"We are going to establish the freedom for everyone to choose his or her partner for a future together."
UMP MP Philippe Gosselin said the legislation was only the beginning of a trend that the French people did not want.
"Today it is marriage and adoption. Tomorrow it will be medically assisted conception and surrogate mothers."
It is expected that the legislation will reach the statute 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.