A mob of about 15 to 20 youths reportedly beat and robbed two people downtown, witnesses told NBC Chicago.
The attack happened near the intersection of West Chicago Avenue and North Wabash Avenue at about 8 p.m.
"When we saw the first punch thrown at the kid, me and my friends took off our uniforms and just darted over there trying to help," said witness Cory Bynum. "We get there, [the victim was] coughing up blood, his face is like really bloody and he's all, 'Oh, they just stole $350 from me.'"
The thieves then ran toward the nearby Chicago Red Line stop, at Chicago and State, another witness said.
Bynum remained on the scene and said he was able to help police identify one of the attackers.
When contacted by NBC Chicago, police said they were investigating a "battery in progress" in the area.
The reported incident comes within hours of police issuing a community alert in connection with a pair of mob attacks on Chicago Transit Authority buses and weekend attacks in Streeterville and the lakefront.