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.
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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.