Police News Affairs Officer Amina Greer said a 20-year-old man on a bicycle approached the off-duty officer while he was walking around 12:30 a.m. Friday, the Chicago Tribune reported Friday.
The man brandished a knife and announced a robbery, leading the off-duty officer to reveal himself as a police officer.
The off-duty officer took the knife-wielding man into custody "without incident," Greer said.
Officers patrolling the area responded to the off-duty officer's call for backup and took the suspect to the Town Hall District station.
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