Bikers converged on Union Sq. last Friday evening for the monthly Critical Mass ride. There was one difference this time, however — there were thousands of them. Mostly in their teens, 20s and 30s, they came from all over to participate in what was billed as a bikers’ bloc against the Republican National Convention. Charging that the bikers were blocking traffic, police made over 200 arrests, many during a notable confrontation outside St. Mark’s Church in the Bowery on Second Ave. The ride was led by the nonprofit environmental group Time’s Up! In response to the charge that they were blocking traffic, the group said, “We are traffic.”