It happened around 11:40 a.m. at Bryant and Lafayette avenues in the Hunts Point section.
According to an eyewitness, the 33-year-old off-duty officer got into an argument with several men, which then escalated into a physical altercation.
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Police say there were at least six people in the group, and one of them began assaulting the officer, possibly with a bat or 2x4.
The witness noted that he did not know the man being assaulted was an officer, as he was wearing plainclothes.
The officer was taken to Jacobi Medical Center and was said to be conscious and alert.
The investigation is ongoing into what prompted the fight.
No arrests have been made.
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