Ringo Starr and Victoria Beckham were among the stars who celebrated at Sir Elton John and David Furnish's civil partnership party.
Liz Hurley, Cilla Black and Lulu were some of those in a celebrity traffic jam outside the lavish party at the couple's Old Windsor mansion.
Sir Elton, 58, earlier "wed" Mr Furnish, 43, at Windsor Guildhall.
Crowds had gathered for their ceremony, which came on the first day of same-sex unions in England and Wales.
Paparazzi and locals were treated to a good view of celebrities as they arrived at the party.
Their cars had to queue through country lanes near the couple's home.
Sitting in the back of a Rolls Royce, Beckham cradled a parcel in silver packaging and chatted to a friend.
Fashion house head Donatella Versace and model Claudia Schiffer were also among the guests.
England cricket captain Michael Vaughan and tennis star Greg Rusedski were there from the world of sport.
Colleagues from the music industry, singers James Blunt and Jamie Cullum were also in the queue.
Entertainer Lulu pointed at the traffic and said: "This could only happen for Elton."
Sir Elton and Mr Furnish were among 700 gay couples joining in a civil partnership on Wednesday.
Similar ceremonies took place first in Northern Ireland on Monday, then in Scotland on Tuesday.
The couples are granted new rights in areas such as employment, pension and inheritance. But the partnerships are not officially regarded as marriages under UK law.
A small number of guests attended Sir Elton's ceremony, including the British video artist Sam Taylor-Wood.
Ozzy and Kelly Osbourne were among the party guests
Ms Taylor-Wood - who arrived with husband Jay Jopling, owner of the White Cube Gallery in London - entered the Guildhall with Arthur, Mr Furnish's black-and-white dog, on a lead.
After the ceremony, the happy couple were showered with rice on the steps of the Guildhall. Two young women managed to break through the police cordon to present them with an apple ice-cream cake.
Asked how it went, Sir Elton said: "Great, thanks."
"It was a very emotional service," Ms Taylor-Wood told the BBC News website.
Lulu was one of the many stars who got caught up in the queue
"Everyone clapped, and of course they kissed at the end," added Mr Jopling.
Actor Sir Antony Sher was also celebrating after he and his long-term partner, theatre director Greg Doran, had their own civil partnership ceremony at Islington Town Hall in north London.
"It's wonderful," Sir Antony said. "It's a little bit of history being made.
"I feel very proud this is happening in this country and that we can be part of the very first group to have it done.
"It means an enormous amount to be legal together after 18 and a half years."