Human rights campaigner Peter Tatchell today delivered an early wedding card to Buckingham Palace calling on Prince William and Kate Middleton to "express their support" for same-sex marriages.
A group of gay activists waved pink Union Jack flags as the giant card was unveiled for the Equal Love campaign.
They have launched a legal bid to overturn the bans on same-sex civil marriages and heterosexual civil partnerships in the UK.
Mr Tatchell said he hoped the royal couple would "find a way" to back the protest.
Some of the eight couples who in February filed an application in the European Court of Human Rights to end the legal prohibitions on same-sex marriage and opposite-sex civil partnerships were also present.
Mr Tatchell said: "We're here to send our best wishes to William and Kate. We wish them a happy wedding day and a happy married life together.
"Kate and William are lucky - they can get married. Gay couples can't - that's not fair.
"In a democratic society marriage should be open to everyone."
He added: "We hope that within the confines of royal protocol somehow Kate and William will find a way to express their support for marriage equality, so that same-sex couples can have the same right as they have to get married if they wish.
"I'm sure Kate and William have gay and lesbian friends. I'm sure they wouldn't wish to see them discriminated against."
Mr Tatchell also criticised the decision to invite several "royal tyrants" to the wedding.
Although the Crown Prince of Bahrain is no longer attending, Mr Tatchell described his original invite as "truly shocking".
He added: "There are seven other royal tyrants on the guest list from countries like Saudi Arabia, Swaziland, Qatar and Brunei. That is very offensive to the victims."