Police have arrested a man over an alleged assault on Leona Lewis at a book signing.
A Scotland Yard representative said police were called to Waterstone's bookstore at Piccadilly, London, after a report of assault.
2006 X Factor winner Lewis did not need hospital treatment, but was visibly shaken and left the signing immediately afterwards.
Lewis' spokesman said the singer had been signing autographs for around an hour-and-a-half when "a guy came up and punched her to the side of the head".
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In a statement, Waterstone's said: "There was an incident at our Leona Lewis signing this afternoon and the event was immediately stopped.
"The security all jumped on him and they were trying to pull him out and he was just laughing
"We are now doing everything we can to support the police in their inquiries."
Lewis had been signing copies of her new autobiography, 'Dreams', which is due to be released next month.
One fan at the scene told the BBC: "My brother saw the whole incident. He [the attacker] walked up there with the book, she signed it and, as she looked up, he just punched her.
"He [my brother] actually heard the impact."
Another eyewitness added: "She was running out with her hand over her eye and I just saw a man on the floor.
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"Suddenly the security all jumped on him and they were trying to pull him out and he was just laughing, he thought it was funny."
Lewis's spokesman added: "The police were called immediately and medical attention has been sought.
"Leona's understandably badly shaken and apologises to the fans she was unable to meet and complete signings for."
Waterstone's stated this was the first incident of its kind it had encountered.
The spokesman add: "We would like to thank Leona for her professionalism and we are extremely sad that this event was ruined for her, and her fans, many of whom queued from the early hours of the morning to meet her"
Lewis' second album, 'Echo', is scheduled for release in November, following the massive popularity of 'Spirit' which was nominated for three Grammys.