1) I'd quickly position myself in a prime, tactical position.
2) I'd yell that the cops are on their way. Most of their attention would be on me. The "real" bad one's would probably still be doing what they were doing.
3) If that didn't cause them to stop, I'd be shooting the one's who I deemed as the main perpetrators of the group.
4) After one shot, I can absolutely guarantee you that they'd all be running. Heck I probably wouldn't even HAVE to shoot anyone. Just one warning shot itself would do the trick.
5) These aren't your typical hardened criminals. They aren't going to be waiting around or coming at you. Just a bunch of punks looking for an easy target with most not wanting to get involved themselves at all and only feel safe because they're in such a large group.
This sounds like a lot, but it would all happen in a matter of not too many seconds.
If you want to get real simple, down to the nitty gritty, this is all it would take:
Draw gun, yell freeze. If for some bizarre reason, this outrage doesn't end right then and there, start taking them out.