In Portland, Oregon, they have a "Critical Mass" bicycling ride every month. The Portland ride apparently attracts up to 800 bicyclists and part of the point, it would seem, would be to promote bike usage over car usage.
Typically the ride would be policed by about 5 cops who would just make sure there was half-decent order, but at some point the city decided to take a harder line. It might have been the fact that many of the CM folks were also at the anti-Bush rally which preceded the last ride, and both sides were able to culture their animosity towards each other.
In any case, the cops decided to really crack down, with pepper spray, rubber bullets, etc. and wound up arresting 9 people and issuing 47 citations.
Apparently the problem was that the bikers were doing things like going through red lights. Once in a while a ride would include a few abusive people. Well hell, let's go crack some heads.
A full pictorial is available
here.
Also a
Portland Tribune story