Several couples were conducting ceremonies on Friday night in a bid to be the first to say "I do", in what Prime Minister David Cameron said was an "important moment for our country".
"Put simply, in Britain it will no longer matter whether you are straight or gay -- the state will recognise your relationship as equal," Cameron said in a statement.
Couples were vying to make history by saying "I do" in the first few minutes of Saturday morning, when the historic marriage law passed by parliament last year came into force.
Many more plan to exchange vows in the hours that follow, including one couple who will perform almost the entire ceremony - barring the legal declaration - in song for a television program entitled "Our Gay Wedding: The Musical".
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 demanded full equality with heterosexual couples.
"We didn't want to get married until it was a marriage that my mum and dad could have, that other men or women could have," Teresa Millward, 37, told AFP.
She is marrying her girlfriend of 11 years, Helen Brearley, in Yorkshire in northern England on Saturday morning. Both will be dressed in cream, and Millward's father will walk them both down the aisle of the register office.
Gay marriage was championed by Prime Minister David Cameron's government, and a rainbow flag will fly over government offices in London all weekend in celebration.
Although members of Cameron's Conservative party and the established Church of England opposed the move, it has support from the majority of the public.
The law is the final victory in a long battle stretching back to the decriminalisation of homosexuality in England in 1967.
"Finally, after years of campaigning, any couple who wants to get married can get married," said Deputy Prime Minister Nick Clegg.
"Together we've made our country a place where we celebrate love equally, gay or straight - and for that reason we should all be raising a glass."
