Prime Minister David Cameron has hailed the first gay weddings in England and Wales as sending a "powerful message" about equality in the UK.
The law changed at midnight, with a number of gay couples vying to claim the title of being among the first to be married as ceremonies took place across the country.
Despite facing opposition from some in the Conservative Party about his backing for the change, Mr Cameron said the reform was necessary because "when people's love is divided by law, it is that law that needs to change".
Writing in Pink News he said "this weekend is an important moment for our country" because "we will at last have equal marriage in our country".
The Marriage (Same-Sex Couples) Act was passed in JulyHe said: "The introduction of same-sex civil marriage says something about the sort of country we are.
"It says we are a country that will continue to honour its proud traditions of respect, tolerance and equal worth. It also sends a powerful message to young people growing up who are uncertain about their sexuality.
"It clearly says 'you are equal' whether straight or gay. That is so important in trying to create an environment where people are no longer bullied because of their sexuality - and where they can realise their potential, whether as a great mathematician like Alan Turing, a star of stage and screen like Sir Ian McKellen or a wonderful journalist and presenter like Clare Balding."
Among the first couples set to take advantage of the legalisation were actor Andrew Wale, 49, and guesthouse owner Neil Allard, 48, who wed at the Royal Pavilion in Brighton just after midnight.
Andrew Wale (L) and Neil Allard said their vows in BrightonFollowing the ceremony in which the couple wore matching suits, Mr Allard said they felt "privileged" and "humbled".
Mr Wale added: "It's kind of extraordinary. We did not really expect it to happen so suddenly, so soon.
"We were waiting and nothing seemed to be happening so we were considering a civil partnership, which is not true equality. So we are very happy this day has come finally."
Peter McGraith and David Cabreza, who have been together for 17 years, tied the knot at Islington Town Hall, north London, just moments after midnight struck.
Ahead of their ceremony, Mr McGraith said: "We are thrilled to be getting married. It is a mark of significant social progress in the UK that the legal distinction between gay and straight relationships has been removed.
Mr McGraith (L) and Mr Cabreza were wed just moments after midnight"Very few countries afford their gay and lesbian citizens equal marriage rights and we believe that this change in law will bring hope and strength to gay men and lesbians in Nigeria, Uganda, Russia, India and elsewhere, who lack basic equality and are being criminalised for their sexual orientation."
Broadcaster Sandi Toksvig and her civil partner Debbie Toksvig will renew their vows at a public event at the Royal Festival Hall on the Southbank in London later this morning.
Same-sex couples wishing to marry had initially thought they would have to wait until the summer after the Marriage (Same-Sex Couples) Act was passed last July.
However, they were allowed to register their intention to marry from March 13 - with March 29 the first day they could get hitched.
Unlike civil partnerships, same-sex marriages will give the same legal recognition as marriage across a range of areas including pensions, inheritance, child maintenance and immigration rights.
While the change will not be enforced upon religious organisations, they will be able to opt in.