Amateur footage showed a person being held on the ground
Leona Lewis has spoken of her "horrible shock" after an alleged assault at a book signing in London on Wednesday.
In a statement she said it had left her "extremely hurt and upset" and also thanked fans for their support, saying it had been "truly overwhelming".
Police have arrested a man over the incident on Wednesday at Waterstone's bookstore in Piccadilly, London.
Lewis's spokesman said she had been signing autographs for 90 minutes when "a guy came up and punched her."
Lewis was publicising her forthcoming autobiography, Dreams, at the event.
The security all jumped on him and they were trying to pull him out and he was just laughing
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.
"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 said it took security at book signings "extremely seriously" and this was the first incident of its kind it had encountered.
Alexandra Burke on Leona's attack
"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," it added.
Fellow X Factor winner Alexandra Burke told BBC Breakfast she had been in touch to offer her support to Lewis.
"I think it's awful and I don't know why someone would do this unprovoked," she said.
"But when things like that happen, it can knock your confidence. I hope that doesn't happen to Leona."
Lewis's second album, Echo, is scheduled for release in November.
The three-time Grammy Award nominee, released her debut single A Moment Like This, shortly after winning the ITV1 show.
She is due to play a gig in her native Hackney in east London on 2 November.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.