At about 7:30 p.m., the man was walking with two females when he got into a verbal altercation with another man near East Monroe Street, police said. The exchange became physical and the offender pulled out a handgun and shot the man in the head. Police said the shooter left the scene on a bicycle.
The victim was taken to Northwestern Hospital in critical condition.
A person of interest was in custody late Saturday, police said.
Anthony Guglielmi, a Chicago police spokesman, said the dispute possibly started over a woman.
The victim's sister, Shelli Fabbri, said after the man shot her brother, he came back and fired more shots. When the man left on a bicycle, Fabbri said she chased after him.
"I chased him through the street, trying to hold the bike down, and his girlfriend comes and punched me and ripped my shirt and I wind up on the ground," Fabbri said.