LONDON — Gay couples in Britain waited decades for the right to get married. When the opportunity came, some had just days to plan the biggest moment of their lives.

Londoners Sean Adl-Tabatabai and Sinclair Treadway registered their intent to marry on March 13, the first day gay couples could sign up for wedding ceremonies under Britain’s new law. Eager to be part of history, the two men picked the earliest possible moment – just after midnight Friday, when the act legalizing same-sex marriage took effect.

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The rainbow flag, a symbol of lesbian, gay, bisexual, and transgender community, flies over a building next to Nelson's Column in Trafalgar Square in London on Friday to mark the start of same-sex weddings in the UK on Saturday.

Wee-hours wedding ceremonies like theirs are taking place across England and Wales, while Scotland follows in the fall. It’s a sign of a profound shift in attitudes in a country that little more than a decade ago had a law on the books banning the “promotion” of homosexuality.

“Some people say, ‘You gays are trying to redefine marriage,’ but the definition of marriage has already changed,” said Treadway, a 20-year-old student originally from Los Angeles. “Now it’s between two people who love each other.”

Most Britons agree. Polls show about two-thirds of people in the country back gay unions, and support is highest among the young. Britain has seen none of the large street protests against gay marriage that have taken place in France.

Same-sex marriage has been welcomed with enthusiasm by Britain’s Conservative-led government. Rainbow flags went up over two government buildings Friday in what Deputy Prime Minister Nick Clegg called “a small symbol to celebrate a massive achievement.” Many local authorities offered midnight weddings, competing to hold the first ceremonies.

Treadway and Adl-Tabatabai, a 32-year-old TV producer from London, will be married in front of 100 guests at London’s Camden Town Hall, before emerging to the strains of “I Got You, Babe” by Sonny and Cher. Guests will travel by double-decker bus to the late-night reception, where they will enjoy drinking, dancing and that most British of dishes: curry.