The off-duty police officer who was critically injured early Saturday morning by “friendly fire” underwent his third surgical procedure yesterday, as police released a videotape of the assault that took place just before his shooting.

The videotape of the assault was taken from security cameras at the White Castle restaurant on Webster Avenue in the Bronx, where the incident took place. In it, five men can be seen taunting Officer Eric Hernandez, 25, and then beating him to the ground, throwing food at him, and kicking him as he crawled out of the restaurant. After the assault, one of the attackers can be seen wearing the officer’s baseball cap and waving his food in front of the restaurant window.

According to police, it was after the assault that a restaurant employee dialed 911 and responding officers found Officer Hernandez, wearing jeans and a white jacket, holding one of his assailants at gunpoint in the restaurant parking lot. A responding officer, who apparently didn’t know Officer Hernandez was an off-duty police officer, subsequently shot him several times, police said. Officer Hernandez was brought to St. Barnabas Hospital, where he was listed in critical condition last night.

In an effort to answer how the “friendly fire” occurred, police officials last night described the initial assault on Officer Hernandez as a “bad beating” that could have impaired his hearing, possibly meaning he did not pick up the on-duty officer’s command to drop his weapon. Officer Hernandez, who has been on the force since July 2004, was assigned to the 52nd Precinct.

Amid press reports that placed Officer Hernandez at several bars prior to the incident, some said the officer’s Fourth Amendment right to privacy was violated when his blood alcohol content was tested, and others speculated that the officer may have been drunk.

“You are a victim of assault. What gives the Police Department the authority to check your blood alcohol?” Lieutenant Robert Gonzalez, the vice president of the National Latino Officers Association and an active-duty Bronx transit cop, said.

Police yesterday would not say whether the officer’s blood alcohol content was measured. Meanwhile, hospital officials defended such testing.

“Any trauma patient that comes in for emergency care undergoes a battery of 40 tests,” including alcohol content, which could have an impact on the effectiveness and safety of blood transfusions or medication given during emergency medical care, a hospital spokesman said.

Officer Hernandez was in “very critical condition” yesterday after he was shot once in each leg and once in his abdomen, the spokesman said. Over the weekend, Mayor Bloomberg and Police Commissioner Raymond Kelly visited the hospital.

While he could not predict the outcome of Officer Hernandez’s treatment, the spokesman said his was a “very, very serious situation” and that the officer’s femoral arteries were severed during the shooting.