Lady GaGa Crowd Surfing at Lollapalooza

Lady Gaga was a headliner at Lollapalooza over the weekend, and tens of thousands of people jammed Chicago's Grant Park to see her perform. But they might not have gotten the best Gaga show of the festival.

A few hours before she took the main stage on Friday (Aug. 6), Gaga made a surprise appearance on a side stage during the band Semi Precious Weapons' set. Or, rather, on a stage and in the crowd.

As you'll see in the (not especially SFW) video above, after joining Semi Precious Weapons on stage and banging on the drummer's kit for a short time, Gaga -- wearing fishnets and pasties -- decided to take a dive into the crowd. Security was not especially pleased with her crowd-surfing, but she seemed to be having a heckuva time. And the crowd, as you might expect, pretty much went nuts. (Check out all the phones and cameras pointed at her when she dives in.)

She also made out a little with SPW singer Justin Tranter, just for good measure, and he joined her in the crowd as well.

From the video, it looks like the SPW set drew a pretty big crowd -- several thousand, probably. We're guessing, though, that many, many more will claim they were there.