France has became the 14th country to legalise same-sex marriage after President Francois Hollande signed it into law following months of bitter political debate.
Hollande acted a day after the Constitutional Council threw out a legal challenge by the right-wing opposition, which had been the last obstacle to passing the bill into law.
The legislation also legalises adoption by same-sex couples.
French Justice Minister Christiane Taubira, who steered the legislation through parliament, has said the first gay marriages could be celebrated as early as June.
But opponents of the measures have vowed to continue their campaign, with a major protest rally scheduled for May 26 in Paris.
The issue has provoked months of acrimonious debate and hundreds of protests that have occasionally spilled over into violence.
Hollande made "marriage for all" a central plank of his presidential election campaign last year.
On Friday, in the wake of the Constitutional Council ruling, he warned that he would tolerate no resistance.
"I will ensure that the law applies across the whole territory, in full, and I will not accept any disruption of these marriages," said the president.
It was "time to respect the law and the Republic", he added.