A 27-year-old policewoman suffered a two-inch cut to her forehead when she was assaulted with an iron bar.
It happened as two officers tried to break up a brawl outside the Donkeyman's club in Spital Hill, Sheffield, at 0415 GMT on Friday.
The injured officer was treated at the Northern General Hospital in Sheffield before being discharged.
Four people were arrested and were questioned before being released on police bail.
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