French President Francois Hollande was set to sign a gay marriage and adoption bill into law on Saturday after it was cleared by the Constitutional Council which turned down a challenge by the right-wing opposition.
Hollande, who had made "marriage for all" a key election pledge, made the announcement saying it was "now time to respect the law and the Republic" after the top French institution cleared the bill on Friday.
The first gay wedding can be held 10 days after Hollande signs it into law.
On Saturday, France will become the 14th country to legalise same-sex marriage, joining a club of eight other European nations -- the Netherlands, Belgium, Spain, Norway, Sweden, Portugal, Iceland and Denmark.
The bill was approved on April 23 by parliament but was immediately challenged on constitutional grounds by the main right-wing opposition UMP party of former president Nicolas Sarkozy.
The UMP had opposed what it said was a fast-track voting procedure and argued that gay marriage represented such a fundamental change that more than a law is required.
The UMP has not made clear whether it would seek to repeal the law if it comes to power. French media have reported that some in the party believe that would not be legally possible.
A statement by the Council said same-sex marriage "did not run contrary to any constitutional principles," and that it did not infringe on "basic rights or liberties or national sovereignty."
However, the council said that gay adoption did not automatically mean the "right to a child" and that the "interest of the child" would be the overriding factor in such cases.
Harlem Desir, leader of Hollande's ruling Socialist party, said: "It's a victory for the French republic and for equality. It's a day of great pride for the Socialists."
But a Parisian socialite who goes by the name of Frigide Barjot and has become the public face of the movement opposing gay marriage said the council's go-ahead was a "provocation."
Barjot whose assumed name is a play on the name of French film star Brigitte Bardot, a 1960s sex symbol, and translates as Frigid Loony, told AFP: "It's an institutional revolution," adding: "We are in the process of changing civilisation."
Gay rights groups hailed the decision as a watershed.
The inter-LGBT association said it sent a "strong message to French society", while its spokesman Nicolas Gougain exulted: "Now it's celebration time."
Leading gay rights watchdog SOS Homophobie added: "Our country has taken a great step forward today although it's regrettable that it was taken in a climate of bad faith and homophobic violence."
Helene Mandroux, the mayor of the southern French city of Montpellier -- known to homosexuals as the "French San Francisco" -- said she was readying to host the country's first gay wedding "in a couple of days."
The issue of gay marriage has divided France, which is officially secular but overwhelmingly Catholic, with street protests against the bill drawing hundreds of thousands and often sparking violence.
The reform initially seemed to enjoy solid majority backing among French voters. But recent polls have suggested the opposition campaign has shifted opinion to the extent that the electorate is now fairly evenly split on both gay marriage and adoption.
France's INSEE statistics agency says about 200,000 people declared themselves as living as same-sex couples in a 2011 study.
The clearance of the bill came on the International Day Against Homophobia and coincided with the release of a European Union report which said that two-thirds of Europe's lesbian, gay, bisexual and transgender community are still afraid to show their sexuality in public.
It said a quarter have been victims of physical or verbal attacks. The online survey, described as the largest of its kind, questioned around 93,000 people in the EU's 27 member states plus Croatia, which is to join the bloc in July.