Here is some info on the video
http://www.nypost.com/seven/07292008...eye_122136.htm
It seems like the police were working a special detail and they encountered the guy. They say he was drunk, swung an umbrella at them and then he started punching them so they took him down. The guy says he was at a party, wasn't drunk and went outside to get some ice where he encountered the cops and they told him he couldn't go back inside because he was drunk. When he told them he wasn't drunk they attacked him.
The truth, as per usual, is probably somewhere in the middle.