Lakefront Trail Mobs - Stories? Encounters?

I was riding north on the trail between Oak St. Beach and North Ave. Beach and I encountered multiple HUGE roaming packs of young, high school-looking kids blocking the trail up. They were jumping in front of bikers and roller bladers and throwing stuff. I remember hearing about lakefront summer mob action the last few summers, but it slipped my mind last night. 

There was a police presence, but it didn't seem effective. This morning I read two stories about more "flash mobs" on the Gold Coast and downtown. 

http://www.chicagotribune.com/news/local/breaking/chi-police-probin...

 http://www.chicagotribune.com/news/local/breaking/chi-cops-man-beat...

Anybody else having run-ins with these? I'm doing a story about the re-emergence of these mobs for Vocalo.org, a Chicago Public Radio station, and I'd love some cyclist voices.