“The incident shows the pro-gay rights movement in Hong Kong has created a chilling effect,” Leung said at a press conference with the groups.

“The advert is highly controversial and can affect the growth of children. As public organisations, [the MTR and the authority] should be socially responsible and maintain their original decision of banning the advert.”

Leung, also an associate professor of law at City University, said the U-turn was caused by “political pressure” from the pro-gay rights movement.

She will also use her position in the Legislative Council to push for protection for businesses, so they have the right not to display advertising they find offensive.

Roger Wong Wai-ming, convenor of one of the 16 groups that signed the joint statement, urged the two firms to consider banning the advert again. He also hit out at a suggested sexual orientation discrimination ordinance, saying it could result in a “pro-gay rights movement monopoly”.

On Saturday, pro-gay rights group Big Love Alliance and some supporters gathered on the westbound platform of Central MTR station to celebrate the unveiling of one of the posters produced for Cathay Pacific Airways.

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Former Labour Party lawmaker Cyd Ho Sau-lan, who showed her support, criticised Leung’s stance.

“Discrimination against the LGBT community is not a right, I cannot emphasise this more,” Ho said.

“People like Priscilla Leung, who dare to teach law at university and get enough votes to sit in Legco, have no idea about human rights or equality.”

The rail operator started putting up the poster on Friday after initially blocking the advert, which it said could “offend against the generally accepted standards of public decency or the social or cultural standards of society”.

The posters appeared at Causeway Bay, Central, Admiralty, and Quarry Bay metro stations, and by Friday evening one had been spotted at Central, one of the busiest stations on the MTR network.

But, despite Cathay buying the entire eastbound platform advertising spots on the busier Island line, the advert was instead displayed at the far end of the westbound platform. By Saturday, the poster occupied three spots in the busier location.

The poster has yet to be displayed at the airport, and the alliance’s chief campaigner Brian Leung Siu-fai said he hoped to see that happen “sooner or later”.

Adverts for the LGBT-friendly Pink Dot event, and the Hong Kong Gay Pride Parade event, have previously been publicised without objection by the MTR Corp.

Betty Grisoni, the Pink Dot co-director, who joined the celebrations, said it was interesting the Cathay advert had been banned, and pointed out that the rail operator allowed other adverts that could offend.

Give it time, Hong Kong will get there on LGBT rights

“We also believe some MTR advertisements are offensive,” she said. “I am not a prude, but some [adverts] have diversity issues or blame people who are overweight, so this was a wider censorship issue we couldn’t let go.”

Her co-director Abby Lee said the poster helped raise the profile of the LGBT community, and showed it was OK for same-sex couples to hold hands in public places such as the MTR, which was for everyone and “love is for everyone”.

Also joining the celebration was Howard Chan Ho-wan, who appeared in the advert.

“It stirs some feelings in my heart,” he said. “With such a big public response fighting back against the opposition, I find it touching and I thank people for that.”