The UK held its first gay marriage ceremonies on Saturday, as a law authorising same-sex marriage came into effect at midnight, the final stage in a long campaign for equality.
David Cameron, the prime minister, hailed what he said was an "important moment for our country", and a rainbow flag flew above government offices in London in celebration.
Put simply, in Britain it will no longer matter whether you are straight or gay
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".