The projection by the gfs.bern polling agency for Switzerland's public broadcasters showed the measure passing by 64% to 36%.

Switzerland's Parliament and the governing Federal Council supported the “Marriage for All” measure, and pre-referendum polls showed solid backing. Switzerland has authorised same-sex civil partnerships since 2007.

Supporters said passage would put same-sex partners on equal legal footing with heterosexual couples such as by allowing them to adopt children together and facilitating citizenship for same-sex spouses. It would also permit lesbian couples to utilise regulated sperm donation.

Opponents believe that replacing civil partnerships with full marriage rights would undermine families based on a union between one man and one woman.

At a polling station in Geneva on Sunday, voter Anna Leimgruber said she cast her ballot for the “no” camp because she believed “children would need to have a dad and a mom.” But Nicolas Dzierlatka, who said he voted “yes,” acknowledged that while same-sex marriage strays from “so-called” tradition, “I think what's important for children is that they are loved and respected — and I think there are children who are not respected or loved in so-called 'hetero' couples.”

The campaign has been rife with allegations of unfair tactics, with the opposing sides decrying the ripping down of posters, LGBT hotlines getting flooded with complaints, hostile e-mails and shouted insults against campaigners, and efforts to silence opposing views.

Switzerland, which has a population of 8.5 million, is traditionally conservative and only extended the right to vote to all its women in 1990.

Most countries in Western Europe already recognise same-sex marriage, while most of those in central and Eastern Europe don't allow wedlock involving two men or two women.

Supporters say it could still be months before same-sex couples can get married, mainly because of administrative and legislative procedures.

Another issue on Sunday's ballot was a measure spearheaded by left-wing groups to raise taxes on returns from investments and capital such as dividends or income from rental properties in Switzerland as a way to ensure better redistribution and fairer taxation.

?The projection showed that proposal failing, with 66% voting against it in a country known for its vibrant financial sector and relatively low taxes, and as a haven for many of the world's richest people.