Flags reading "Demo for all" are seen during a protest against same-sex marriage on February 2, 2013 in Marseille
France's National Assembly overwhelmingly approved a key piece of legislation that will allow homosexual couples to marry and adopt children, to the delight of gay activists.
Deputies voted 249-97 in favour of Article One of the draft law, which redefines marriage as being a contract between two people rather than necessarily between a man and a woman.
Although the proposed law still faces at least another week of scrutiny before a final vote scheduled for February 12, it now looks set to emerge from parliament without delay and undiluted.
After months of frenzied debate, the ease with which the law cleared its first major hurdle was welcomed by delighted gay rights activists.
"Things are going well and quickly, which is a relief for us," Nicolas Gougain of the Inter-LGBT lobby group told AFP.
"We've never seen so many deputies arguing the case for equality and for the recognition of different kinds of families. We can now look forward to the remainder of the debate calmly. It's fabulous!"
But as activists celebrated, tens of thousands of opponents to the law gathered in cities across France on Saturday.
Police estimated that 80,000 people took to the streets nationwide. The conservative Paris suburb of Versailles saw the biggest turnout, with 15,000 demonstrators counted -- more than twice as many as in the capital itself.
The article approved on Saturday was supported by deputies of the ruling Socialist Party, who enjoy an overall majority in parliament, 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."
The National Assembly also rejected a clause that would give mayors the right to refuse to marry a gay couple.
But 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 a widely felt concern among opponents of the government's plans.
Paris Archbishop Andre Vingt-Trois claimed Saturday that moves to legalise surrogacy would logically follow the adoption of the legislation currently before parliament.
"What we predicted is already starting to happen," the cleric claimed, citing a government circular which recommends that the children of gay couples who have used the services of a surrogate mother outside of France should be granted French nationality.
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, many of which were mobilised by the Roman Catholic Church, have underlined that those who oppose gay marriage feel strongly about the issue.
President Francois Hollande has been accused of pushing the legislation through without proper consultation.
Many of the estimated 6,500 people who protested in Paris were outside the National Assembly, joined by members of the UMP party.
UMP president Christian Jacob warned of the "ethical and bioethical disorder" the law would bring while another party member hoped that a narrower Socialist majority in the Senate could halt the bill altogether.
Opponents are gathering 500,000 signatures to present to lawmakers with the aim of blocking the legislation. They plan another rally in Paris on March 24.
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.