This video reportedly shows Los Angeles police menacing riders in a “critical mass” bike protest against British Petroleum by sticking out their feet. According to the cyclists, when one person filming the scene objected, he was arrested.
The protesters say that an officer was trying to either kick or trip a cyclist when another cyclist shouted out an objection. The next thing shown is an officer taking down the photographer.
One of the concerns with the video is that it seems to follow a worrisome trend of police arresting anyone filming them on public streets, including some police departments who have said that any filming of police in such circumstances is a crime (here and here). In England, police appear to be arresting people on the same grounds, here.