WATCH: Plainclothes NYPD officer repeatedly strikes teen already subdued by three other cops
NYPD officer strikes teen (YouTube)

A plainclothes New York Police Department officer is being investigated by Internal Affairs after a video in which he rushes up and attacks a teen surfaced, the Daily News reports.

In the video, three uniformed officers can be seen subduing the suspect while another uniformed officer supervises when a plainclothes officer in a camouflage jacket rushes toward the suspect and begins to punch him, repeatedly, in the stomach.

"Stop it," yelled bystander Sarah Doneghy, who recorded the arrest and uploaded the video to YouTube.

"He's 12!" shouts another bystander. "Why would you just do that? After everything that's happened! I'm a lawyer -- I'm writing all this down."

"Like he would come up from behind like it on a blonde-haired, blue-eyed kid," she continued. "That's acceptable to you, because he's an animal? That's acceptable to you? There's no way you'd treat a white kid like that."

Police said that the two teens were accused of assaulting a man with a cane.

"What did I do?" one of the teens asked as he was being placed in a car. "Can I hear what I did? Can you say what I did?"

"Take it easy," a bystander tells the teen as he's being arrested. "Don't give anybody a reason to do anything."

Watch the entire eight minute video of the confrontation below.