The singer was promoting her new autobiography Dreams at a Waterstone's in Piccadilly, London, when a man is said to have approached her.
'Leona was the victim of an unprovoked attack this afternoon at her book signing,' reads a statement from her spokesman.
'The police were called immediately and medical attention has been sought. Leona is understandably shaken and apologises to the fans she was unable to meet and complete signings.'
An emotional Leona, 24, was ushered out of the venue by security and was snapped in tears as she got into her car. She later pulled out of an appearance on The One Show.
'There was an incident at our Leona Lewis signing this afternoon and the event was immediately stopped,' reads a statement from Waterstone's. 'We are now doing everything we can to support the police in their enquiries. We would like to thank Leona for her professionalism and we are extremely sad that this event was ruined for her, and her fans.'
A Scotland Yard spokesman confirmed a man has been arrested on suspicion of actual bodily harm.
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