I'm following the progress of Critical Mass on anoise.org right now -- streaming ant-RNC news all weekend.

Good background from Newsday on Critical Mass (emphasis mine):

Hundreds of bicyclists who flout traffic rules could face arrest tomorrow night when police crack down on an unauthorized ride through midtown.

The monthly ride, Critical Mass, has for years proceeded without police intervention even as thousands of riders blocked streets, ignored red lights and took up traffic lanes. Under the law, bicyclists must obey traffic laws, have a working bell and are banned from riding more than two abreast.

...

The ride is scheduled the last Friday of each month and is meant to encourage pollution-free transportation. It has no preset route, instead following the whim of its leaders, who are not chosen beforehand. It is slated to begin in Union Square at 7 p.m. tomorrow and end at a West Side bar.

A police spokesman would not say why officers avoided ticketing riders for the more than 10 years that Critical Mass has been holding rides. During a ride in June, 1,000 bicyclists took up both northbound lanes on Park Avenue on the way to the South Bronx, ignoring traffic signals. A few riders blocked intersections until the group passed through.