The 'Love The Way You Lie' star was greeted by hundreds of screaming fans at the Westfield mall as she took to the stage to celebrate the beginning of the holiday season.
She told the event's host George Lamb that her Christmas wish was "more sleep" and joked to the crowd that they should add her new album Loud to their Christmas lists. She added: "I just want to wish you all to get the gifts that you want."
The 22-year-old was joined on stage by a group of school children for the moment when she switched on the lights, which was accompanied by fireworks and festive music.
She later posted on her Twitter account: "Westfield Mall, London! Its BANANAS in here! #RihannaNavy it doesn't get better than u guys (sic)."
Loud is released in the UK on November 15 and in the US the next day.