Thailand has passed a landmark marriage equality bill, becoming the first country to approve same-sex unions in conservative Southeast Asia.

The Thai senate on June 18 approved the measure in its final reading, with 130 senators in favour, four against, and 18 abstentions.

Overjoyed LGBT people and their supporters are seen in the video celebrating at the Bangkok Art and Culture Centre to mark the occasion.

Activists gave emotional speeches lauding the historic decision, which is seen as a 'monumental step forward' for equal rights for those pursuing alternative sexual and relationship lifestyles in Thailand.

There were also lively performances by drag queens and Thai singer Tachaya Prathumwan, followed by a thanksgiving ceremony to cap off the party.

The bill is set to be endorsed by the king and will take effect 120 days after being published in the royal gazette.

This makes Thailand - which despite the move still has barbaric free speech laws and notoriously corrupt public bodies - the first country in Southeast Asia and the third in Asia, following Nepal and Taiwan, to legally approve same-sex marriage.

The bill will grant alternative sexuality couples the same legal rights as straight couples, including adoption, inheritance, and health care decision-making rights.