Gay couples across England and Wales said "I do" on Friday night as a law authorising same-sex marriage came into effect at midnight, the final stage in a long fight for equality.
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, 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".
They were among several couples bidding to be first to take advantage of last year's Marriage (Same Sex Couples) Act by holding midnight weddings.
"We are very happy this day has come finally. It's very exciting," said Wale, a 49-year-old theatre director.
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".
Civil partnerships have been legal since 2005 and marriage brings no new rights - the ability to adopt, for example, was introduced in 2002.
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.
"This weekend is an important moment for our country," the prime minister wrote in an article for Pink News.
"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 percent agreed gay marriage should be permitted, with 26 percent 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.
The House of Bishops last month also warned clergy they should not bless married gay couples.
But Archbishop of Canterbury Justin Welby, the leader of the world's 80 million Anglicans, said the Church had accepted the new law and would continue to demonstrate "the love of Christ for every human being".
Peter McGraith and David Cabreza, who have been together for 17 years, also married shortly after midnight in front of friends and their two adopted sons in London.
They hope their wedding will send out a message to places like Nigeria, Uganda and Russia where the idea of gay marriage is a distant dream.
While 15 countries have legalised gay marriage and another three allow it in some areas, homosexuals remain persecuted in many parts of the world.
"There's a lot of gay men and lesbians around the world who are not invited to the party," McGraith, a clothing designer, told AFP ahead of the big day.
Same-sex couples who were married abroad are now recognised under the new law, although only in England and Wales.
Scotland, which has devolved powers, is expected to introduce gay marriage later this year, while the British-controlled province of Northern Ireland remains deeply divided on the issue and has no plans to change the law there.