"We're absolutely delighted," said 80-year-old John Walton, registering in London with his partner of 40 years, Roger Raglan. "It's enormously important to us that we should be able to state to everyone that we are partners."
Among the first to register were pop star Elton John and his filmmaker partner David Furnish, whose official proclamation was posted alongside those of other impending weddings and partnerships at Maidenhead Town Hall west of London. After the mandatory 15-day waiting period, the couple plan a private ceremony in nearby Windsor, where they have a home.
The law, passed last year despite some opposition from Parliament's unelected House of Lords, gives same-sex couples the same social security, tax, pension and inheritance rights as married ones. Prime Minister Tony Blair's center-left government dropped the word "marriage" from its legislation rather than run afoul of legislators who feel the word has religious connotations.
The first ceremonies will be held on Dec. 19 in Northern Ireland, Dec. 20 in Scotland and Dec. 21 in England and Wales.
The Times newspaper marked the day by publishing notices of "gay marriages" for the first time.
One of the partnerships announcements in the newspaper was placed by Graham Ferguson, 67, and Christopher Heyd-Smith, 59, a retired couple from Lyme Regis in southwest England, who plan a civil partnership ceremony on Jan. 30.
"We have been living together very happily for 34 years, we have our wills made out properly to benefit each other, but we feel it is a privilege to be able to have our partnership legally recognized," the newspaper quoted Ferguson as saying.
While the legislation aroused some opposition, it did not provoke a huge controversy. It caps a remarkable transformation in social attitudes that began when Victorian laws outlawing homosexuality were overturned in England and Wales in 1967, although they persisted in Scotland until 1980 and Northern Ireland until 1982. In 2000, the government lifted a longstanding ban on gays serving in the armed forces and lowered the homosexual age of consent to 16, the same as for heterosexuals.
"Britain has been in the dark ages over this but today we have made the first step into the 21st century," said Percy Steven, registering his partnership with Roger Lockyer at Westminster Council House in central London.
"When we first started living together, we were breaking the law," said Steven, 66.
"If somebody had said to me that one day gay people would be able to have their partnerships recognized, I would have said 'Yes, but not in my lifetime."'
Up to 1,000 couples were expected to register their partnerships on Monday. In Brighton, the south coast beach resort that is Britain's self-styled gay capital, the register office opened at 7:30 a.m. to accommodate several couples who were already waiting.
"It was wonderful," said the Rev. Debbie Gaston, a pastor at Metropolitan Community Church in Brighton who plans to formalize her union to partner Elaine Gaston on Dec. 21. "It was very emotional, very special. We were aware that it was history in the making and we were overwhelmed by it all. It's been a long time coming.
"I believe it's God that has opened up this door for the civil partnerships to happen."
The Netherlands, Canada, Belgium and Spain have legalized same-sex marriage, while Germany, France and Switzerland have similar laws to Britain. In the United States, Massachusetts alone allows gay marriage, while Vermont and Connecticut permit civil unions.