Katy Perry stunned fans by making a surprise appearance at a sneak preview of her new movie in London on Thursday.

The "I Kissed A Girl" singer showed up unannounced to introduce "Katy Perry: Part Of Me" to a theatre full of excited youngsters in the British capital.

She told the crowd, "Hey, hello! Is it alright that I'm here? We're going to watch my movie!" before popping on a pair of 3D glasses and taking a seat in the middle of the cinema."

Perry also posed for pictures with fans and handed out popcorn to the audience.

The singer posted a picture of herself sat in the cinema on her Twitter page and wrote, "Had tons o' fun in London today at a surprise screening... Wish I could sit w/ (with) all of u (sic) when u see (the film)."

The documentary, which opens in July, features concert highlights and behind-the-scenes footage, as well as clips from her personal life and childhood.