Lucian Capellaro/Courtesy Katy perry
's fans were in for a big surprise on Wednesday night when they attended a screening for the pop star's Paramount documentary, Katy Perry: Part of Me
Clad in a red floral dress with an open back, the "Wide Awake" singer popped up
at the theater inside The Grove in Los Angeles just as the lights went down for the viewing to begin.
Once Perry arrived, she told the crowd, "You guys are all so cute, even all the way to the back," an onlooker tells PEOPLE.
"Little girls rushed the screen as Katy walked around slapping hands and greeting fans," the source says. "She managed to sign some autographs and pose for a few pictures before trying to disperse the massive crowd that had gathered around her."
Perry, who said a few more sweet things to her admirers on the mic, "was all smiles and seemed genuinely excited for her young fans to see the film," the insider says, adding, "The start of the screening was delayed a good 10-15 minutes before Katy sat down in the middle of the screaming crowd for a bit." – Jennifer Garcia