More clashing between apparent Trump supporters and Black Lives Matter protesters -- and, per usual, more of the former being left on their asses after taking a whooping.

This fistfight went down Friday at a demonstration in Pittsfield, MA -- where BLM protesters were facing off with what appear to be Trump supporters ... which led to these two fellas going at it. You see the one dude in red -- who seems to have stumbled out of the Trump crowd -- start to square up with the younger guy in a BLM-emblazoned shirt.

They throw a few punches, which mostly miss at first, but eventually ... the more spry of the two lands a hit that takes the Trump-er down, and hard. He hits the pavement and lays motionless on the ground as the other guy walks away. Looks like a clean knockout.

Pittsfield PD tells us they're investigating this further, adding an off-duty cop who was nearby arrived on the scene quickly. The man who took the punch was more or less okay.

We're told he was taken to a hospital with minor injures -- per reports, he was supposedly alert and conscious at the time. The police haven't yet made any arrests.