When the front of the Mich Ave march was close to State & Wacker, the cops told us that the back of the march was at Pearson Street (one block north of Chicago) -- this was confirmed by one of our demonstration facilitators a little while later. That's roughly 12 blocks. With a conservative estimate of 1,000 people per three lanes of traffic per block, that works out to 12,000 people. At one point the cops were saying 20,000 - 30,000 people, but we think that may have been an excuse to justify getting more Dept. of Homeland Security dollars -- the gold rush of the early 21st Century.