Police say eleven juveniles and one adult were taken into custody for recklessness and obstructing traffic.
"There was like a huge riot going on - someone held up a phone and the police knocked it out of her hand - that's all i saw, though. I'm scared for my safety," said witness Martha Nulu. "I never know what's gonna happen, and I usually come downtown thinking it's a safe area so ... kind of scary."
Police say there were no fights or other violence connected with Saturday night's arrests.
Those arrests come on the same day a new state law was passed to crack down on people who use social media to organize groups for so-called "flash mobs."
Governor Pat Quinn signed the bill into law Saturday along Michigan Avenue.
It comes in response to an uptick in such crimes in that area in recent months.