BANGKOK: Thai lawmakers met on Tuesday (Jun 18) to vote on legalising same-sex marriage, putting the kingdom on the cusp of becoming the first Southeast Asian nation to recognise marriage equality.

The Senate upper house is expected to approve the legislation, after which it will go to King Maha Vajiralongkorn for royal assent and come into force 120 days after publication in the official Royal Gazette.

Thailand would become only the third place in Asia where same-sex couples can tie the knot, after Taiwan and Nepal, and activists are hoping the first weddings could be celebrated as early as October.

"Today is the day that Thai people will smile. It is a victory for the people," Tunyawaj Kamolwongwat, an MP with the progressive Move Forward Party, told reporters.

"Today it finally is happening in Thailand."

Tunyawaj, one of the leading voices pushing for equal marriage in parliament, posed alongside fellow MPs and aides with a rainbow banner.

Senators began their session at 9.30 am (10.30am, Singapore time), and a vote to give final approval to the law is expected in the afternoon.

The new legislation changes references to "men", "women", "husbands" and "wives" in marriage laws to gender-neutral terms.

It also gives same-sex couples the same rights as heterosexual ones when it comes to adoption and inheritance.

Prime Minister Srettha Thavisin, who has been vocal in his support for the LGBTQ community and the bill, will open his official residence to activists and supporters for celebrations after the vote on Tuesday.

Activists will later hold a rally, featuring a drag show, in central Bangkok, where giant shopping malls have been flying the ra...