Switzerland voted by a wide margin to allow same-sex couples to marry in a referendum on Sunday, bringing the Alpine nation into line with many others in western Europe.

Official results showed the measure passed with 64.1per cent of voters in favour and won a majority in all of Switzerland's 26 cantons, or states.

Switzerland's parliament and the governing Federal Council supported the Marriage for All measure. Switzerland has authorised same-sex civil partnerships since 2007.

Supporters said the passage would put same-sex partners on an equal legal footing with heterosexual couples 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 voted yes, said what children need is love.

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, he said.

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 emails, 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.

Also on Sunday, voters dismissed a proposal 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.

Results showed 64.9per cent 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. No canton voted in favour.