Nadeem Sajad contacted police and asked them to move on a large group of disorderly youths who had congregated outside his newsagents store in Avenue Road, Kings Heath.
Police failed to respond to the call, and one hour later an underaged member of the group assaulted shop assistant Abdul Rehman for refusing to sell him cigarettes.
Abdul suffered severe bruising to his face and eye and had to be taken to hospital for treatment.
He is now refusing to return to work in the store.
Nadeem said: “He is very shaken up. The whole situation could have been avoided because I rang police more than an hour earlier. I’m not happy with them – they cannot be busy all the time.”
West Midlands police are now investigating the incident. A spokesman said: “Police in Kings Heath had to prioritise a number of emergency calls during a busy period that evening and therefore officers were unable to attend the newsagents.
“It is our aim to take positive action against antisocial behaviour and as a result of this matter patrols in the area have been increased.”