David Cameron has hailed the first same-sex marriages in England and Wales as sending a "powerful message" about equality in Britain.
The law changed at midnight, with a number of gay couples vying to claim the title of being the first to be married in Britain by trying to time it perfectly so their vows were said just seconds after the clock struck 12.
The Prime Minister 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".
As gay and lesbian couples across England and Wales say I do, they will be in good company as broadcaster Sandi Toksvig renews her vows with Debbie Toksvig.
The free but ticketed event in the Royal Festival Hall on London's Southbank forms part of the centre's celebrations of the Marriage (Same Sex Couples) Act becoming legal from midnight.
The Radio 4 News Quiz presenter said the event would be a "joyous occasion" and invited the public to share the experience, at which they will joined by their four children.
Toksvig, who had a civil partnership with Debbie in 2007, said: "I look forward to celebrating with old and new friends. It is a public statement of pride in our love and thanks to all those who have campaigned for equality over many years."
MPs and councillors across England and Wales are celebrating the inauguration of the first same-sex marriage ceremonies taking place at midnight today. Yvette Cooper, Emily Thornberry and Terry Stacey attended ceremonies this evening.
As the countdown for the first same-sex marriages gets underway, Deputy Prime Minister Nick Clegg, former Green Party leader Caroline Lucas and shadow chancellor Ed Balls have hailed the momentous event.
Ed Miliband has joined Nick Clegg in welcoming the first same-sex marriages, which take place just after midnight tonight.
In a message for Pink News, the Labour leader said it was an "incredibly happy time" for same-sex couples as well as "an incredibly proud time for our country".
The Archbishop of Canterbury has signalled a shift in tone in the Church of England's response to same-sex marriage, which will become law at midnight tonight.
Although the church has been divided over the new law, Justin Welby struck a conciliatory tone in his comments.
"I think the church has reacted by fully accepting that it's the law, and should react on Saturday by continuing to demonstrate in word and action, the love of Christ for every human being."
One of the first same-sex couples to get married in England have told Daybreak their wedding is "a big step forward" and "a great privilege".
Andrew Wale and Neil Allard from Brighton said just getting married was more important to them than being the first gay couple down the aisle.
Trevor Love, the registrar officiating the UK's first official same-sex marriage, has told Daybreak how the wedding is "the most exciting day of my life".
Andrew Wale and Neil Allard are to get married in Brighton Pavilion at midnight.
Deputy Prime Minister Nick Clegg said the legalisation of same-sex marriage was "a great step forward".
"It's a landmark day," he said. "I am so chuffed about this change in the law."
"It's just a simple idea that if two people love each other it doesn't matter that they're the same sex - if they love each other and want to show that commitment towards each other through marriage, they should be able to do so."
Same-sex couples will be able to wed from the stroke of midnight tonight as the new law permitting same-sex marriage comes into force.
As the clock strikes 12, gay couples will be able to wed in the first ever ceremonies in England and Wales.
A number of couples are vying to claim the title of being the first ever to be married in Britain by trying to time it perfectly so their vows are said just seconds after midnight.
The Marriage (Same-Sex Couples) Act came into force in July last year.