On the 29th of January, a "friends of Abe" event in NYC featured infamous right-wing liar James O'Keefe (of Veritas fame). When Huffington Post writer Ryan Clayton backed by other activists took to the stage to confront O'Keefe, O'Keefe's thugs responded by choking him, dragging him down the stairs, and stepping on his head. Another progressive infiltrator, Jason Carter, ended up in the hospital with injuries after also being assaulted.
The assault scene that was captured near the end of the Huffington Post video
The entire Huffington Post video, showing how singing a song precipitated the assault(Youtube 5:43)