Several gay couples in England and Wales flocked to get married as soon as the clock struck midnight on Friday.
The first same-sex marriages have taken place in various places across the region as gay marriage became legal in the two countries at midnight, reports suggest.
British Prime Minister David Cameron hailed the marriages as a "powerful message" about equality in Britain. Many politicians from the major parties have also hailed the change in the law.
Scotland had passed a similar law in February while the first same-sex marriages are expected to take plane there in October. Northern Ireland, however, has no plans to follow suit.
In an article from the Pink News website, the Prime Minister Cameron stated: "This weekend is an important moment for our country.
"The introduction of same-sex civil marriage says something about the sort of country we are. It says we are countries 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 re equal' where straight or gay. That is so important in trying to create an environment where people are no longer bullied because of their sexuality," he added.
After many couples were reported to have got married by morning, Cameron tweeted: "Congratulations to the gay couples who have already been married - and my best wishes to those about to be on this historic day."
Congratulations to the gay couples who have already been married - and my best wishes to those about to be on this historic day.
— David Cameron (@David_Cameron) March 29, 2014
"Every wedding has a photographer but this one had hundreds," remarked Jo Black from BBC News.
"Outside Islington Town Hall, well-wishers stopped in the steet to point their smartphones while TV crews fought for the best views. As the loving couple appeared on the steps, crowds cheered and even sang the tune to Here Comes The Bride -ironic perhaps, since this was a gay wedding with two grooms," he said.
Peter McGraith and David Cabreza, who have been partners for 17 years, were one of the first same-sex couples to marry in London on Saturday.