L.A. Sheriff's Deputies Beat Detained Compton Man, Think No One's Watching

A man was wrestled to the ground by cops, and then momentarily got the snot beat out of him ... before a vigilant eyewitness called them out and forced them to stop.

The outrageous scene was shot in Compton, CA, earlier this week, and it shows L.A. County Sheriff's deputies attempting to detain a guy from behind some type of wall, sorta hidden from plain view. Somebody is filming from afar, unbeknownst to the officers.

At first, it looks like the guy's resisting -- but eventually ... they get him on the ground, and he makes it clear that he's not fighting them anymore. Not enough for these deputies though, it seems, 'cause at least one more rushes over and they start to pound on him.

It's crazy too ... before the beating -- which involved fists and knees -- one of the deputies looks around, as if to see if anyone is watching them. A few seconds into the thrashing, somebody else aside from the cameraman calls out, asking why they're whooping on him.

That forces all the deputies to halt immediately, and that's where the video cuts out.

LASD tells us, yes, these are their guys ... and an investigation into the use of force by the deputies is underway. If misconduct is found, we're told a personnel complaint will be filed.