Tinley Park Mayor Ed Zabrocki said police do not know if attackers were connected to the NATO Summit demonstration in downtown Chicago.
"The guys we have in the lockup are still not talking, at least not this morning," Zabrocki told the newspaper.
The owner of the Ashford House restaurant in Tinley Park said 18 young men stormed into the dining room and attacked patrons with hammers and batons.
Tinley Park police told The Chicago Sun-Times they had five suspects in custody Saturday.
Mike Winston said he was working in the kitchen when waitress screamed that a fight had broken out. The suspects -- whose faces were covered -- overturned tables and broke dishes, in what police called a targeted attack. Winston said the suspects went straight for a table of some 20 diners from out of state.
"They came running in the door single file," he said. "Once they attacked the table, they went and started hitting random people."
Winston chased after them, and had one of them "on the ground, then five guys got out of the car and started kicking the [crap] out of me," he said. He suffered several bruises, and was the only restaurant employee to be hurt.