A powerful video posted to YouTube shows a bloodied man in the moments after he had two teeth knocked out by masked vandals late Tuesday in Oakland during a protest against police brutality that ranged from Berkeley to Oakland to Emeryville. (Warning: profane language.)

The video, posted by former Chronicle reporter Justin Berton, shows the aftermath of the unidentified man’s efforts to stop people from lighting a dumpster on fire. At a number of recent protests, some demonstrators have sought to stop others from turning peaceful rallies into stages for window-smashing, graffiti, looting and violence. Sometimes they chant “Peaceful protest,” and in some cases they actually try to step in, which can be dangerous.

As the victim in the video retrieves his glasses from the dumpster, a man tells him, “You need to let everyone do their own thing,” a reference to destructive protest tactics. “You can’t block anyone from doing their own thing.” Then a woman on a bike swears at the injured man, calls him an idiot and tells him, “Go the f— home.”

Resolved to make his point, the man says, “This harms movements. This harms our ability to protest. Every time we do this, they can stop our marches … They just knocked my two teeth out. I did nothing violent …. All I did was passively try to stop them.”

“Don’t do it,” another man at the scene tells him. “Look what happened.”

UPDATE: Another video, posted to LiveLeak, shows the actual attack on the victim as he tries to stop the burning of the dumpster and is surrounded by people berating him and telling him he is “acting like police” at an anti-police protest. (Warning: graphic violence and profane language.)

This wasn’t the first time a protester has been badly injured in the past week after trying to keep others peaceful. On Sunday, a man who tried to stop window-smashing at a Radio Shack in Berkeley took a vicious blow to the head, an episode also caught on video. (Warning: graphic violence.)