Switzerland will hold a referendum on legalising same-sex marriage, the government confirmed on Tuesday.
Switzerland is one of the few remaining countries in Europe where same-sex marriage is not legal.
The Swiss parliament approved a bill in December allowing same-sex couples to marry.
However, under Switzerland's direct democracy system, new laws can be challenged and put to a referendum if more than 50,000 valid signatures are collected within 100 days.
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Opponents in the wealthy nation of 8.6 million people gathered more than 60,000 valid signatures, the government said in a statement.
The Swiss normally vote three to four times a year on a wide range of topics at the national, regional and local levels - either laws challenged by petition or proposals that have gathered enough signatures.
A date for the vote, described as the "referendum against 'marriage for all'," has not yet been set.
The law's opponents, including members of the Swiss People's Party (SVP) - the country's biggest - and the marginal, similarly right-wing populist Federal Democratic Union, had said they would try to trigger a referendum.
The SVP said it was "intolerable to want to place marriage on an equal footing with any form of cohabitation".
Same-sex couples can register a civil partnership in Switzerland.
However, this status does not provide the same rights as marriage, including for obtaining citizenship and the joint adoption of children.
The two chambers of parliament approved the bill in December after multiple rounds of debate spanning back to 2013.
The wording of the bill allows gays and lesbians to marry and for lesbians to access sperm donations.
