Pop star Sir Elton John, 58, and Furnish, 43, sealed an 11-year relationship on the first day ceremonies became legal in England and Wales.
The low-key ceremony was attended by the pair's parents, including Sir Elton's mother Sheila.
Artist and photographer Sam Taylor-Wood entered the Guildhall with Arthur, Mr Furnish's black-and-white dog, on a lead.
The happy couple left the half-hour ceremony wearing matching diamond rings.
Victoria Beckham was among the throng of celebrities who attended the lavish party to celebrate Sir Elton and Mr Furnish's union.
It gave locals and the paparazzi a chance to see who, like Cilla Black, made it on to the guest list.
Actress Liz Hurley looked demure in white as she arrived with her partner Arun Nayar.
Kelly Osbourne was left waiting in the traffic jam with the rest of her family - mum Sharon, brother Jack and dad Ozzy.
Supermodel Claudia Schiffer was also at the party, alongside her film director husband Matthew Vaughn.
Other guests included Lulu, singer Joss Stone, Hollywood actress Sharon Stone and jazz star Jamie Cullum.
An official portrait of the couple taken by Ms Taylor-Wood has been released.