In Chicago, a massive gathering of some 10,000 people was held at the Federal Plaza, followed by a seemingly spontaneous march up Lake Shore Drive during rush hour. The march was originally in the northbound lanes, but the march spilled into the southbound lanes also stopping traffic going both ways. At some point, marchers at the front decided to jump the dividers and head over to Michigan Avenue (the touristy Magnificent Mile). The police who had been mild-mannered until then began to get afraid of something akin to the World Trade Organization protests in Seattle in 1999 happening and started to call for back-up and block the protesters from reaching Michigan Avenue. The march was eventually blocked at Chicago Water Tower from going down Michigan Avenue. The police closed the protesters in from both sides and wouldn't let them move or leave. At this point, the group of protesters had dwindled down to 600 or so. After an hour, many protesters wanted to leave but were unable to. They began chanting things like: "Let us leave. We have to pee." Every ten minutes or so, three or four police officers would go into the crowd and beeline for a certain person (possibly those the police believed were organizers) who they would grab and arrest. The police arrested more than five hundred protesters, and detained several hundred more.