The radio station, which was also celebrating its first year on air, hosted the main stage where one of London's most-loved bands, Madness, performed.
Additional entertainment included catwalk shows, stilt walkers, roaming musicians, a ferris wheel, carousel and children's rides.
Two giant birthday cakes made of thousands of cup cakes provided a focal point, while outside Hamleys there was a children's lawn offering face painting, animal characters, a birthday tea party, balloon artists and magicians.
The event raised money to support Cancer Research UK's vital research into the prevention, diagnosis and treatment of cancer.
To see pictures of the Regent Street Festival click here.
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