Singer Leona Lewis is recovering after being punched in the head as she signed books for fans.
The 24-year-old was signing copies of her autobiography at Waterstone's Piccadilly store in London on Wednesday afternoon, when a man in the queue suddenly lashed out.
She was bruised at the side of her head and was left "shaken", her spokesman said.
Police arrested a 29-year-old man at the scene. He remains in custody.
Lewis was taken to see a doctor as a precaution after being struck, but needed no further treatment.
Witnesses said she had been happily signing autographs for about 90 minutes when a man whose book she autographed suddenly punched her in the side of the head.
Her spokesman, who was at the signing, did not hear the man say anything to Lewis during the "unprovoked" incident and security officers swooped in. He was seen laughing immediately afterwards.
The spokesman said: "(Leona) is absolutely gutted she did not get the signings finished." He added that to hit a woman was "beyond cowardly".
Waterstones said it took security at book signings "extremely seriously" and this was the first incident of its kind.
The firm added: "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."
