Sydney man robbed by cop impersonator

A young man has been robbed by a police impersonator in Sydney's inner west.

The 21-year-old was walking along Piper Street in Leichhardt on Sunday morning when a black sedan pulled up beside him, with the male driver hopping out and demanding to see some identification.

The thief, who claimed to be a plain-clothed officer, took the man's phone and wallet back to his vehicle to allegedly perform checks but instead drove off.

Police are appealing for any witnesses to come forward.