The attack went down Thursday night as demonstrators took to the streets again to protest the killing of George Floyd and police brutality ... the latter of which was on full display.

Take a look -- as cops appear to be trying to clear protesters from the area, one of them snaps and bum-rushes a guy and throws him down ... then starts delivering violent left hooks to his head.

The officer's so unhinged, he even strikes his own guy as he turns around to go at more protesters, before seeking out his original target for more action.

Chicago PD tells us they've opened an investigation into the incident. CPD adds it "strives to treat all individuals our officers encounter with respect. We do not tolerate misconduct of any kind and if any wrongdoing is discovered, officers will be held accountable."

This incident came just hours after one in Buffalo, where cops pushed an elderly man to the ground, causing him to hit his head and bleed -- and the day after NYPD officers assaulted a man they said was violating curfew.