Three men are on the run after allegedly seriously assaulting the officer and another off-duty policeman during the late-night incident in Sydney's  inner west.
Police have launched a major investigation into the fracas, which happened about 11pm on Wednesday at Newtown.
Officers arrived to find a 25-year-old off-duty policeman unresponsive on busy Enmore Rd, which is lined with restaurants and bars.
An off-duty nurse performed CPR until paramedics arrived.
The officer was taken to the nearby Royal Prince Alfred Hospital, where he is in a serious but stable condition.
Superintendent Despa Fitzgerald said both of the officers were from the southwest Sydney command.
"The officer was unconscious for some time and without (CPR) assistance we may be talking about something else," she said on Thursday.
The two off-duty officers were walking along the road when they were approached by a group of three unknown men, who were arguing with each other.
Both officers were then assaulted and one was knocked to the ground, and repeatedly punched and kicked in the head, police said.
The two policemen had not yet been interviewed after the assault and investigators were yet to determine if they had identified themselves as officers.
Enmore Rd was designated as the first "special entertainment precinct" in NSW in late 2023. (Neve Brissenden/AAP PHOTOS)
"The CCTV shows a group of men attacking an officer that is on the ground and another officer, and it is very violent," Supt Fitzgerald said.
"I have watched it, it's very confronting and distressing."
Police Minister Yasmin Catley said the incident was difficult to watch.
"This is a despicable act against two officers who were out enjoying themselves in this very vibrant part of our city and we now have an officer who is still in hospital," she said.
The three men ran from the scene before police arrived.
Investigators are looking for two men of Maori or Pacific Islander appearance, and a third man of Caucasian appearance, all between 20 and 30 years old.
Enmore Rd was designated as the first "special entertainment precinct" in NSW in late 2023, recognising the area's popularity as a late-night music and hospitality hub.
The designation allows extended trading hours for certain venues and looser noise controls for businesses.
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