Gah, MRD beat me to it.

Here's another source, complete with wide angle shots of the snow ball fight (showing no political signs), and a commenter insisting the people were anarchist types:
What this article fails to mention is that there were several left wing/anarchist types at this snowball fight and that it was meant to be a political protest as much as a snowball fight. People with masks and bandanas around their face (and various anti-war/anti-capitalism signs) were pelting cars as they passed. One happened t be a plain-closed detective. He got out with a walkie talky (HE WAS NOT THE ONE WITH A GUN) and called for patrolmen to respond. After being called every imaginable obscenity and getting hit with many snowballs, one to the face, other officers arrived, one of these patrol had his gun withdrawn (not pointed at anyone) and told the group to break up. As soon as he saw there was no immediate threat the officer holstered his gun, even as he was hit with snowballs himself

CR