Australia has launched a postal vote on whether to legalise same-sex marriage.
The non-binding and voluntary ballot of nearly 16 million people will run until November.
An Ipsos/Fairfax opinion poll has suggested that 65% of eligible voters intend to take part in the survey – and of those, 70% said they would support gay marriage.
If the postal survey finds that most Australians are in favour, Parliament will vote on legislation to lift the prohibition on gay marriage.
However, several lawmakers have warned they will vote against such a law – irrespective of public opinion.
On Sunday, 20,000 people took to the streets of Sydney in support of a Yes vote, occasionally meeting opposition demonstrations bearing signs in favour of “traditional” and “natural” marriage.
Opposition leader Bill Shorten, who supports same-sex marriage, told the Sydney rally that the change was about Australia’s “identity as a nation”.
He said: “Do we believe in equal care for all, equal rights for all? We can win this thing!”
The campaign against changing the law has focused on issues beyond marriage itself, including gender education in schools and the “right” of children to have a mother and a father.
A video created by the No campaign shows a mother expressing concern that her son was allowed to wear a dress to school, and claims “in countries with gay marriage, parents have lost their right to choose”.
The postal vote, which offers a yes or no response to the question of whether the law should be changed to allow same-sex couples to marry, was called by Malcolm Turnbull’s conservative government.
The ballot’s unusual format was decided after the election promise of a national plebiscite was rejected twice by parliament’s upper house.
Mr Turnbull has urged Australians to vote Yes, telling reporters that “the right to marry is a conservative ideal as much as any other principle”.
Two past conservative prime ministers, Tony Abbott and John Howard, have publicly opposed the change.
The vote will close on 7 November, with the result being released on 15 November.