The event, which saw the road completely closed off to traffic, was the 57th annual Christmas Lights Switch-On event at Oxford Street, and marked six weeks until the big day.
The day event included balloon-making, craft workshops, face-painting and a grotto for children, and culminated in the light switch-on in the evening.
Turning on the lights, the 'Born to do it' star said: "Growing up as a kid, me and my mum and dad at home, was like 'whose gonna turn them on this year?' so to have the opportunity is beautiful.
"There's so many people who are less fortunate.
"Children who are out there, who are having a hard time.
"Those people are the people that we need to be focusing on and let's try to make a difference to those people as well."
The comeback singer was joined by X Factor winner Louisa Johnson, and boy band The Vamps, who put on fun performances for visitors to the lights-on event.
The mayor of London, Sadiq Khan , also attended the event.
He said: "Christmas doesn't really begin until the lights have been switched on in Oxford Street."
The NSPCC launched its Little Stars campaign at the switch on.
Members of the public were asked to donate £5 to name one of Oxford Street's world-famous lights for someone special in their life, with funds going to the children's charity.
Christmas lights will be switched on throughout west London in November and December.
Click here to find out when the lights on your local high street will be turned on, with more events announced every day.
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