Bell said Lewis, who was launching her new autobiography "Dreams," was shaken up by the incident and went to see a doctor as a precaution.
Lewis shot to fame after winning the "X Factor" reality show in 2006, and her powerful voice has led to comparisons with Mariah Carey and Whitney Houston
London's Metropolitan police said a 29-year-old man was arrested on suspicion of assault and is being held in custody.
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