Thousands gathered at Taylor Square, Sydney on Sunday afternoon to call on the government to allow lawmakers a free vote when Labor’s same-sex marriage bill is introduced to federal parliament on Monday.

Opposition and Labor party leader Bill Shorten become the first major-party leader in the country to put his name to a marriage equality bill.

He had formally given notice of a private member’s bill to propose an Act to amend the Marriage Act 1961 that will define marriage as a union ‘of two people’ rather than a ‘man and woman.’

He says that Australia’s current law excludes some individuals of marriage and to him, it was something ‘unacceptable,’ ABC News reported.

PM Tony Abott’s sister Christine Forster, who is engaged to a woman, was one of the keynote speakers at the marriage equality rally that also included Deputy Opposition Leader Tanya Plibersek, Green’s Senator of South Australia Senator Sarah Hanson-Young, and Rodney Croome of Australian Marriage Equality.

Forster called for a bipartisan approach over efforts to legalize gay marriage as unless the bill received bipartisan support in Parliament it may not even be debated.

Liberal MP Warren Entsch and Labor MP Graham Perrett have announced they will co-sponsor a new marriage equality bill and seek support from members in other parties and from independents.

Abbott, who doesn’t support same-sex marriage, has given a green light to cross-party co-operation and a Coalition debate on a free vote.

‘MPs and Senators are falling over themselves to declare their support’ as Ireland’s recent successful referendum has proven to be the ‘tipping point,’ said Croome, National Convenor of Australian Marriage Equality and one of the founders of the Australian Coalition for Equality (ACE).

‘Its reputation as (being) socially conservative makes Australia look backward. Its adoption of marriage equality leaves Australia as the only developed, English-speaking country that doesn’t allow same-sex couples to marry,’ Croome wrote in a column published by Australia’s Gay News Network.

Shorten on Thursday said he hopes the effort to legalize same-sex marriage will be bipartisan because ‘it is the only way it can succeed.’

‘But the only way that it can succeed is if Tony Abbott allows a conscience vote. That is the question which is before the Parliament.’

The Labor leader added that Australians would be ‘shocked’ if a free vote was denied to Liberal MPs.

According to media reports, he will present his same-sex marriage bill to the Parliament on Monday although that bill is unlikely to force a vote on the issue.

The Sydney Morning Herald quoted Australian Marriage Equality (AME) as saying that it is confident there are now at least 72 lower house MPs who would vote for same-sex marriage, which is just four votes shy of the 76 required to pass a bill.

While in the Senate, AME estimates that 39 currently support marriage equality and just one more vote is needed for it to pass.