Gaga was at a party thrown for her in a New York hotel when a fan took a picture of her with a phone as she came down the hotel stairs.
An onlooker told Page Six: 'One of her over-protective security guards started roughing him up. He pushed the guy up the stairs, grabbed his phone and threw it, then dragged him back up onto the roof.'
Upon seeing the scuffle, the onlooker insisted: 'Gaga told her guard, 'What are you doing? Leave him alone'.'
The source added that Gaga 'looked embarrassed and shook her head' at the ordeal.
A Gaga spokesperson did not get back to Page Six for comment, and an event rep had no comment.
Gaga was honoured with the Fashion Icon Award at the Council of Fashion Designers of America awards gala and the incident allegedly happened at the afterparty.