The officer was bitten in the knee and suffered "significant injury," police said in a news release late Friday.
The officer shot the dog three times, and both the officer and the dog were receiving medical treatment Friday night, police said.
Police responded to a report of a fight between two men in front of a home in the 2200 block of South Pond Street about 8:45 p.m. An officer tried to break up the fight and was attacked by a large dog owned by one of the men, police said.
Officers later learned the dog had bitten the man who was fighting with its owner.
The incident is under investigation and charges are pending. Further information about the condition of the officer and the dog was not available late Thursday.