From Infoshop News: When wearing a mask is a revolutionary act:

January 28, 2002, 12:50 PM EST
NEW YORK -- Bracing for large-scale protests at the World Economic Forum, the NYPD drew a line in the sand Monday over an obscure law barring demonstrators from wearing masks. Chief of Patrol Joseph Esposito told a news conference that the department would strictly enforce the code, which prohibits three or more protesters from wearing masks. "Three or more with masks and they're marching, they're under arrests," Esposito said.