Mainstream media is slow to pick up on this story but reliable multiple reliable twitter sources - independent journalists have posted pictures and are confirming the details.
The basics are ambulance(s) arrived to transport at least two protestors to hospital.
Here are some twitter pictures: Police van just plowed through the crowd of peaceful activists!! #noNATO
Video: Recorded live on May 19, 2012 9:38pm CST by StopMotionsolo
Recorded live on May 19, 2012 9:55pm CST Cop van pushes protester in Chicago by TWRBS
UPDATE: @OccupyChicago Lauren Digioia confirmed that JACK AMICO, AKA MAYHEM, an #OWS protester, WAS RUN OVER by CPD VAN #6751.
UPDATE 2: @Timcast and I think one of the livestreamers that capture the video clip above (TWRBS) and several other indy journalists were detained briefly by CPD and cops attempted to delete footage on phones...
Sun May 20, 2012 at 8:28 AM PT: