Switzerland votes on Sunday whether to legalise same-sex marriage after opponents secured a referendum challenging the government's plans to bring in "marriage for all".
Polls indicate broad support for legalisation, which would bring the Alpine nation into line with most of Western Europe.
Polling stations remain open until noon (8pm AEST), with the result expected to follow within hours.
"Don't miss the opportunity to participate in this historic step towards equality," the Marriage Yes campaign said Saturday, urging voters to the polls.
According to the last survey conducted, 53 per cent are in favour of legalising same-sex marriage, with 27 per cent against.
Market researchers gfs.bern surveyed 13,261 voters between 1 and 9 September.
An earlier gfs.bern survey between 2 and 16 August 2 put the balance of support at 55-20.
The polling showed that members of some Christian congregations and supporters of the right-wing populist Swiss People's Party (SVP) - Switzerland's largest political party - were the most likely to oppose legalising same-sex marriage.
Rights battle
Switzerland decriminalised homosexuality in 1942, but numerous local and regional police forces continued to keep "gay registers", some into the early 1990s.
Same-sex couples can already register a civil partnership, with around 700 such partnerships established each year.
However, this status does not provide the same rights as marriage, including for obtaining citizenship and the joint adoption of children.
After years of debate and discussion, the Swiss parliament approved a bill last December allowing same-sex couples to marry in the country of 8.6 million people.
But it was challenged under Switzerland's direct democracy system, with opponents gathering the 50,000 signatures needed to put the issue to a referendum.
The wealthy Alpine nation looks set to allow same-sex couples to marry. Source: AFP
The government and parliament have urged voters to back "marriage for all", and to eliminate the current "unequal treatment" between heterosexual and gay couples.
If approved, the law change will allow same-sex couples to marry in civil ceremonies, and provide them with the same rights as other married couples.
Foreign spouses will become eligible to apply for citizenship through a simplified procedure, and same-sex couples will be permitted to jointly adopt.
And it would give lesbian couples access to sperm donations, in what has proven the most controversial aspect of the bill.
Stark poster campaign
Opponents have plastered Swiss cities with stark posters decrying the commodification of children and warning the law will "kill the father".
One of the posters shows a crying baby with its ear tagged like cattle, and the question: "Babies on demand?"
Another featuring a huge zombie-like head, meant to represent a dead father, was covered over by a nearby primary school in the southern Wallis canton out of fear it would frighten the children.
Switzerland looks set to allow same-sex couples to marry, and grant them the same rights as their heterosexual counterparts. Source: AFP
A second vote is being held alongside the referendum, on an initiative brought forward by the youth wing of the Socialist Party, entitled "Reduce taxes on wages, tax capital equitably".
Proponents of the so-called "99 per cent" initiative want greater taxation on high levels of capital income, with the threshold to be set by parliament. The revenues generated would be used to reduce income taxes for the less well off.
The gfs.bern poll found that 46 per cent were against the initiative, with 26 per cent in favour.
"Let's fight against the metropolitan champagne socialists by voting twice NO, to the 99 per cent initiative and marriage for all!", the SVP said in one of its final campaign posts.
With AAP
