While at the Pretty Reckless-hosted launch of Tokidoki x Onitsuka Tiger Collaboration in LA, Momsen reportedly lashed out at fans wielding cameras and mobile phones at the concert.
According to E! Online, she said: 'I know this is the modern age and everyone's got cameras and [mobile] phones and video cameras.
'But I'd really appreciate it if you'd put that f****** s*** down.'
The 17-year-old actress-singer added: 'I know it's very cool to videotape and put it on YouTube the next day, but, frankly, it completely takes away from the show.
'I'd thoroughly appreciate it if you'd put that s*** down and put your hands in the air