Prime Minister David Cameron hailed what he said was an "important moment for our country", and a rainbow flag flew above government offices in London in celebration.
In Brighton on England's south coast last night, Neil Allard and Andrew Wale exchanged vows and rings in the opulent splendour of the Royal Pavilion in front of about 100 guests.
Wearing velvet-collared three-piece suits with white flowers in their buttonholes, the smiling couple of seven years hugged and kissed after they became "husband and husband".
"We are very happy this day has come finally. It's very exciting," said Wale, a 49-year-old theatre director.
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Emerging out into the crisp night air after the ceremony, they were stunned to be greeted by hundreds of cheering supporters.
To calls of "speech, speech!", they expressed hope that the change in Britain would help bring equality to same-sex relationships in other countries.
Earlier this week, Wale remarked "how lucky we are to live in a comparatively tolerant part of the world".
But campaigners have insisted that only the right to marry gives them full equality with heterosexual couples.
"We didn't want to get married until it was a marriage that my mum and dad could have," said Teresa Millward, 37, who will wed her her long-term girlfriend on Saturday.
The gay marriage law is the final victory in a long battle stretching back to the decriminalisation of homosexuality in England in 1967.
Cameron backed the change despite strong opposition from members of his Conservative party and the established Church of England.
"Put simply, in Britain it will no longer matter whether you are straight or gay -- the state will recognise your relationship as equal."
Not all attitudes have changed. A poll for BBC radio said 20 percent of British adults would turn down an invitation to a same-sex wedding.
However, the survey also found 68 per cent agreed gay marriage should be permitted, with 26 per cent opposing it.
The Church of England had opposed same-sex marriage, insisting weddings should only take place between a man and a woman, and secured an exemption from the new law.