It happened around 8:30 Friday night at a VFW hall. He had been attending a birthday party.
Eyewitness News sources say the gunman may have entered the VFW with the intention of robbing it, and when the officer tried to intervene, the gunman shot him in the chest.
The officer was rushed to Loyola Medical Center in Maywood in critical condition.
St. Charles Road was closed a block in either direction from the VFW as police searched the surrounding neighborhood with dogs to locate the gunman.
No one is in custody.