This happened during an OccupyWallSt.org march on Saturday September 24 at around 2 PM at the SE corner of Union Square in NYC.
As you can see at around 0:30, a young man in a red shirt, Glenn Daniels Jr, is walking near the sidewalk with hundreds of other protestors. The crowd was attempting to cross the street to continue the march south down Broadway from Union Square. Daniels is peacefully walking with a water bottle, not committing any crime. At 0:35 he is approached by an NYPD officer and pushed towards the sidewalk. At 0:38 a senior police officer in a white shirt quickly approaches and grabs Daniels and another young man with a beard and backpack. The lighter skinned man is let go, but Daniels is arrested. The remainder of the video shows NYPD officers cuffing and detaining Daniels.
This has been the norm since the beginning of the Occupy Wall Street protests, which have been entirely peaceful. I myself was shoved hard by a senior police officer three times during the march today, and it was almost impossible to take video without capturing blatant violations of protestors' first amendment rights.
If you're in NYC please call Mayor Bloomberg at 311 (or 212-NEW-YORK if you're outside of NYC) and demand an end to continuous police harassment of peaceful protestors. Let them know the whole world is watching.
Visit Zuccotti Park in Lower Manhattan or OccupyWallSt.org to find out how to help or get involved in this growing nonviolent movement to take back our democracy from corrupt financial interests.