The officer at the Brooklyn Detention Center Complex was closing a gate on Tuesday when an inmate attacked, beating, punching and kicking the officer and at one point slamming him to the ground.
Eventually other officers came to the aid of their colleague, who suffered serious injuries and remains in the hospital, union officials said.
The incident was captured on camera.
"He definitely has a concussion because he was knocked unconscious and then continued to be pummeled by this inmate," said Elias Husamudeen, president of the Correction Officers Benevolent Association. "And if you watch the video, you see the size of this inmate."
He lashed out at the city, saying not nearly enough is being done to protect his officers.
Separately, several officers were also attacked on Tuesday at Rikers Island.
The Department of Correction says all inmates involved were re-arrested and moved to restrictive housing during the investigations.
"Multiple officers from two different facilities went to the hospital tonight after inmates assaulted them. We are working with the Brooklyn and Bronx district attorneys to make sure that the next facilities these inmates visit are state prisons," Deputy Commissioner of Public Information Peter Thorne said in a written statement.
Union officials responded on Wednesday by calling for the resignation of New York City Corrections Commissioner Cynthia Brann, saying the jails have become overrun with gang members who are running the jails.
"She has proven over the last three years to be in over her head and have no idea of what it means or what it takes to run a New York City jail system," said Husamudeen.
The union representing correction officers says there have been more than 2,00 attacks on officers since 2014, mostly by incarcerated gang members.
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