A paramedic who stopped his car to help a man who appeared to be lying injured in a Sydney gutter was then viciously assaulted, police say.

The 27-year-old was driving to work at 5.20am on Sunday when he saw a person lying on John Batman Avenue in Werrington County.

The paramedic pulled over and approached the man to see if he could provide first aid, only to be punched in the chest by him.

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A second man then approached the pair from behind and also began assaulting the victim, who suffered fractured ribs, internal bruising and grazes in the assault.