Deputies voted 249-97 in favour of article one of the draft legislation which redefines marriage as being an agreement between two people rather than necessarily between a man and a woman.
Although the proposed law still faces at least another week of parliamentary scrutiny before a final vote scheduled for February 12, the ease with which it cleared the first hurdle indicated it is almost certain to emerge unscathed from the debate.
The article approved on Saturday was supported by deputies of the ruling Socialist Party, who enjoy an overall majority in the Assembly, other leftists and Greens as well as at least one member of the UMP, the main centre-right opposition force.
"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 deputy Philippe Gosselin said the government was forcing through legislation that France did not want.
"Today it is marriage and adoption. Tomorrow it will be medically assisted conception and surrogate mothers," he said in comments that reflected the strength of feeling among opponents of the government's plans.
Opinion polls suggest a clear majority of French voters support the right of gay couples to wed and a narrower majority favour them being granted the right to adopt as couples (gay men and women can already adopt as individuals if approved by social services).
Massive demonstrations across the country have underlined that those who oppose gay marriage feel very strongly about the issue and President Francois Hollande has been accused of pushing the legislation through without proper consultation.
The Catholic church has been heavily involved in mobilising opposition and protests were scheduled to take place again Saturday in towns and cities
A national rally in Paris last month attracted hundreds of thousands of demonstrators and was at least twice as big as a protest staged by supporters of the reform.