The grand launch of Regent Street's Christmas lights suffered a technical hitch when the 8,000-bulb display failed to light up.
The Regent Street lights failed to switch on for 20 minutes
Singer Daniel Bedingfield plunged the power lever to light up the street - but nothing happened.
About 10,000 shoppers had gathered to see the event on Thursday, with many more watching live coverage on BBC One.
The display only surged to life after the crowds were kept waiting in the dark for 20 minutes.
A spokesman for the Regents Street Association said he had "no idea" what had gone wrong with the lights.
"They had been tested many times and had no previous problems," he said, adding an investigation was under way.
Bedingfield had earlier entertained the crowd with live renditions of his Gotta Get Thru This and If You're Not The One.
Two Blue Peter presenters then started a dramatic countdown to the switch-on, before the singer was joined by nine-year-old competition winner Leo Thomson at the power controls.
...but all was well in the end when the lights went on
The technical hitch also caused the sound system to fail, leaving Bedingfield to lead acapella renditions of his hits until power was restored.
Afterwards he told BBC London: "I think I'm the only human in history that single-handedly demolished Regent Street lights and the sound system.
"I feel very proud of myself. I've gone down in history tonight.
Opera group Amici Forever and boy band D-Side also performed at the ceremony, which traditionally marks the start of the festive period in London.
Previous guests of honour at the event have included Kylie Minogue, pop idol Will Young and Silence Of The Lambs star Anthony Hopkins.