Posh Spice - singer Victoria Adams - flicked the switch to illuminate Bond Street, one of the city's smartest shopping areas.
Victoria, who follows in the footsteps of Diana, Princess of Wales, Caprice and Ruby Wax in becoming a "Bond girl" for the night, said she was amazed by the turnout.
She said: "I thought I was going to be here alone. It's great that so many people braved the cold weather.
"I haven't bought any presents yet but I'm really looking forward to Christmas."
The Spice Girl, who married Manchester United star David Beckham in the showbiz wedding of the year, turned on the lights at the 'Allies', the Churchill and Roosevelt bronze at the corner of Old Bond Street and Clifford Street.
'Class and style'
The choir from Sevenoaks school in Kent entertained the crowd with Christmas carols.
This year's set of lights - which include giant blue stars along the mile-long street - are brand new after the Bond Street Association agreed to have them sponsored for the first time.
Tova Borgnine, head of American firm Tova Cosmetics, which sponsored the lights, said she was "overwhelmed" by the sight.
The wife of Hollywood legend Ernest Borgnine said: "We don't have anything nearly as grand as this in America. Bond Street is the epitome of class and style."
The switch-on was followed by a glitzy party at Asprey & Garrard, hosted by Edward Asprey, chairman of the Bond Street Association, and Tova Cosmetics.
Dating back to 1686, Bond Street has become synonymous with high fashion and the most expensive designer outlets.
Showbiz
Music
Film
Arts
TV and Radio
New Media
Reviews
London's bright past
(21 Dec 97 | Christmas decorations)
The BBC is not responsible for the content of external internet sites.