Britney Spears Bum-Rushed Onstage at Vegas Concert, Fears Gun (UPDATE + MUG SHOT)

11:12 AM PT -- Law enforcement sources tell TMZ ... the man who crashed Britney's set is 37-year-old Jesse Webb. We're told he was acting disorderly during her performance from out in the crowd, and was asked to leave by security. That's when he jumped onstage and got tackled. He was arrested for trespassing -- no weapons were found on him. Dude looks like a charmer.

Britney Spears got bum-rushed by a guy who is probably hurting this morning, because her dancers and bodyguards annihilated him.

It went down Wednesday night in Vegas at Britney's Piece of Me show at Planet Hollywood. As Britney sang "Till the World Ends" ... a man crept onto the stage and made his move.

Security and the dancers lunged at the guy as Britney seemed oblivious to the danger. You hear her ask if everything is OK. Check it out ... Britney's knee buckled as she clutched a security guard and asked, "He's got a gun?"

The man was subdued and ejected, but not before either kicking or kneeing one of the guards.