Police are investigating an incident during which a motorist was bashed and robbed in Lyons at the intersection of Hindmarsh Drive and Melrose Drive eastbound on Friday morning.
At the scene, an off duty police member sprung into action detaining the offender until police arrived.
While stopped at the intersection at about 9.10am, the 19-year-old passenger of a red Ford Ranger approached a driver of a maroon Toyota Corolla, opening the driver's door and verbally abusing him.
The Ford Ranger passanger then attempted to punch the other driver and grabbed a necklace from the driver's neck.
He then returned to his car with the necklace before the off duty police member jumped in to make the arrest.
Police are appealing for any witnesses to contact Crime Stoppers on 1800 333 000, or via act.crimestoppers.com.au.