Gay marriage will be legalised at midnight in England and Wales
Same-sex couples will be allowed to legally marry in England and Wales for the first time from midnight.
Politicians from the main parties have hailed the change in the law.
David Cameron said the move sent a message that people were now equal "whether gay or straight", but some religious groups remain opposed.
Scotland passed a similar law in February; the first same-sex marriages are expected there in October. Northern Ireland has no plans to follow suit.
In an article for the Pink News website, the prime minister wrote: "This weekend is an important moment for our country."
'Feels safer'
The law change would encourage young people unsure of their sexuality, he added.
"It says we are a country that will continue to honour its proud traditions of respect, tolerance and equal worth.
"We are a nation that is growing stronger economically because of our long term economic plan. But I hope we can also be a country that is growing stronger socially because we value love and commitment equally."
Liberal Democrat leader Nick Clegg praised the legal change, saying "Britain will be a different place" as a result.
He congratulated his party for being part of the reform, saying: "If our change to the law means a single young man or young woman who wants to come out, but who is scared of what the world will say, now feels safer, stronger, taller - well, for me, getting into coalition government will have been worth it just for that."
'Unchartered territory'
Labour leader Ed Miliband congratulated gay couples planning to tie the knot.
"This is an incredibly happy time for so many gay couples and lesbian couples who will be getting married, but it's an incredibly proud time for our country as well, recognising equal marriage in law," he said.
However, he warned that the "battle for true equality" was not yet won.
While a BBC poll suggests a fifth of British people would turn down an invitation to a same-sex wedding, some gay vicars are prepared to lose their jobs for the right to marry.
The Rev Andrew Cain says he does not want confrontation with the Church, but "must do what is right"
Among them is the Reverend Andrew Cain, who said he will not be "frightened" out of doing what is true to him.
Mr Cain said the Church of England was faced with "unchartered territory".
The Church of England has urged priests to support members of the congregation who are in same-sex marriages, but it has ruled that priests themselves must not enter into one.
The CofE is prohibited by law from performing same-sex marriages, and the Roman Catholic Church is also opposed to them.
Mr Cain, who plans to marry his partner in the summer, said he intends to do so whether the Church approves or not.
"They didn't say we weren't allowed to; they urged us not to and there is a debate going on in the Church about the issues of same-sex marriage, and I happen to have a different perspective on that debate than my bishops," he said.
"It is more important to do what is right than to be frightened into not doing what I believe to be true.
"And I won't be frightened by what the possible consequences are for me. I could lose my job, absolutely. Lose my job, my home and my place."
He said that while the Church's bishops had made it clear they were uncomfortable with some of their clergy planning same-sex marriages, he wanted to move the debate on the issue forward and be part of the "progressive" side.
On Friday, Archbishop of Canterbury Justin Welby told the BBC the church would now drop its opposition to the introduction of same-sex marriage in England.
He said Parliament had made a decision and the church had to respect it.
The Roman Catholic group Catholic Voices said the findings of the BBC's survey reflected the reality that people remained "deeply uncomfortable" with the move.
Helen Brearley and Teresa Millward decided a civil partnership would not mean as much as marriage
Long-term couple Teresa Millward and Helen Brearley plan to get married on Saturday morning.
The pair, who will wed in Halifax, have not had a civil partnership because they believed it did not offer the same rights as marriage.
Ms Brearley said the new legislation puts them on a more equal footing with heterosexual married couples.
Ms Millward, her fiancee, added: "The certificate we get on the day will be the same as the certificate that my dad has with his wife, that my mum has with her husband, that my brother has with his wife and that my sister has with her husband.
"There will be no difference, so in that case, there is true equality."
James McCarraher, a toastmaster from Hayling Island in Hampshire, said he agreed with the government's decision to allow same-sex marriages.
"For a truly happy and integrated society, there has to be equality - gay couples should be entitled to marry each other in a modern and progressive society," he said.
'Hypocritical'
David Beaton from Leicester said he agreed with civil partnerships but would not attend a gay wedding. He said he thought getting married in a religious ceremony would be hypocritical.
"If gays or lesbians believe in the Christian, or other faiths, and claim to be following doctrine and the tenets of the church they belong to then it is hypocritical for them to demand a ceremony that is at odds with their own claimed beliefs," Mr Beaton said.
"You can't have it both ways."
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