Shortly after taking the stage, "a man in the audience started heckling Pete," a source tells PerezHilton.com exclusively. "The man was making fun of Pete's hoody, mocking Ashlee Simpson and calling Pete a sell-out."

After performing only a three song set, the band got up and walked off stage, but before Wentz walked out the side door he "he lunged for the heckler in the audience and punched him in the side of the face. A few other guys got some shots in, including a bodyguard and some random guy from the audience. [he's so punk rawk he should tour with Justin Timberlake! - more on this story

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