Check out this video that was captured Saturday down in the East Village, where you see at least 4 plain-clothes officers yank a guy out of a building and throw him to the ground while people around them scream, and one woman insists the dude did nothing wrong.

While they're dealing with him, another man approaches the ruckus from the street, only to be met by a lone cop who gets up and tells him to back up -- brandishing his taser and even zapping it in what seems to be an effort to scare him. The guy didn't back down.

That led to the officer taking him down, putting him in a bear hug ... then slapping and punching him when he was on the ground. Another officer came over to help put cuffs on the bystander. Even more chaos ensues as the crowd grows and backup arrives.

NYPD Commissioner Dermot Shea says this all started with a social distance infraction, which he says his cops were trying to break up. Now, this is being investigated by the department's Internal Affairs unit ... and one of the cops on video is on modified duty now.

Meanwhile, 3 people were arrested in the dust-up -- including the 2 guys taken down by the NYPD as well as a woman who allegedly had her own stun gun on her.

On the flip side ... the other side of town was also congregating en masse Saturday over in the West Village, but there weren't too many signs of rowdy, physical break-ups like we see here. The NYPD says they issued dozens of summons over these social distance breakers too, but we haven't seen video of any violence like these POC are experiencing.