Updated at the bottom with more background on the officer, via the LAPD.

An on-duty Los Angeles police officer beat a man with a baton as the victim was on his knees with his hands on his head, and it was captured on security video, the L.A. County District Attorney's office alleged today.

The cop, identified as 30-year-old Jonathan Lai, pleaded not guilty today to “one count each of assault by a police officer and assault with a deadly weapon,” the D.A.'s office stated.

See also: L.A. Cop Offered to Sell Drugs to Undercover Detective, D.A. Says

The case is unusual in that it's rare for the District Attorney's office, which has to work closely with police to prosecute suspects, to charge a cop for an incident involving on-duty use of force:
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This prosecution signals the continued willingness on the part of elected D.A. Jackie Lacey to go after LAPD officers despite their collective political power in the city.

However, the D.A.'s office says the department actually investigated the case, apparently before bringing it to prosecutors.

A D.A.'s spokeswoman said this appears to be the first prosecution for an on-duty cop's alleged use-of-force crime since October, when Officer Mary O'Callaghan was charged with suspicion of “assaulting an arrestee under color of authority.”

The suspect in that case later died in custody.

See also: Mary O'Callaghan, LAPD Officer, Charged With Assault After Suspect's Death

Lai's alleged assault happened at a restaurant near Staples Center on April 15, 2012. Prosecutors say it captured by the eatery's security video.

According to a D.A.'s statement, here's what allegedly went down:

While on routine patrol on April 15, 2012, Lai and his partner approached a man standing alone outside a restaurant near the Staples Center. The restaurant's security video allegedly shows Lai repeatedly using his police baton to strike the man who was on his knees with his hands on his head.

We asked an LAPD spokeswoman if Lai was still on the force or if maybe he had been assigned to home, but she had no information on the case. 

It's not clear how badly the victim was injured.

The suspect's bail was set at $35,000.

He's due back in court Aug. 21. If successfully convicted, Lai could face four years behind bars.

[Update at 4:58 p.m.]: The LAPD says Lai is a 6-year veteran who was working out of the Central Division downtown when the incident occurred. According to a statement:

Officer Jonathan Lai was assigned to LAPD's Central Division when he and his partner approached a man standing outside a restaurant near downtown's Staples Center. Officer Lai used his collapsible baton to strike the man several times while taking him into custody.

The LAPD conducted a use of force investigation and discovered video footage of the incident. Soon after reviewing the video footage, an internal affairs investigation was initiated and Officer Lai and his partner were removed from field duties pending the outcome of the investigation. LAPD subsequently referred the case to the L.A. District Attorney's Office, Justice System Integrity Division, for possible criminal charges.

Chief Charlie Beck:

The LAPD carefully reviews incidents involving the use of force and we conduct thorough investigations to ensure that officers are held accountable for violating policies and going beyond the law. I am always concerned when an officer is accused of violating the public's trust and we will continue to work closely with the District Attorney's Office and the Department's Office of Inspector General to take appropriate action and work to prevent these types of incidents in the future.

The department says Lai surrendered to fellow cops this morning before he was booked into jail.

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