One man's unconventional marriage proposal to his boyfriend onboard the subway in Beijing has set tongues wagging - in a good way - on the Internet.

Online news site Shanghaiist reported that the proposal took place on a rainbow-decorated train carriage on Line 1 of Beijing's metro system.

A 43-second video posted on YouTube showed the man kneeling down in front of his stunned partner as he popped the all-important question while holding out a watch.

"Today I invite all the people we know and do not know to bear witness," he declared in Mandarin as the majority of commuters in the carriage cheered him on and broke into applause.

There were also calls for them to kiss as they embraced each other after the successful proposal.

Images of the incident have since gone viral in China, with microblogging site Weibo flooded with positive comments and messages of encouragement for the couple.

The original post has been shared more than 10,000 times on Weibo, CNN reported.

One user wrote: "This type of love really needs courage, bless you!"

Gay marriage is not legal in China, although public perception towards rights for gays has shifted for the better. Homosexuality was decriminalised in 1997.

In June, gay dating app Blued tied up with popular Chinese online shopping site Taobao to send 10 gay couples to the US to get hitched legally.

A gay couple also held their wedding ceremony in Beijing, a day after the US legalised same sex marriage in all 50 states on June 26.