Chicago crowd size pics
Occupy Chicago's estimate:
Crowd estimators use a simple multiplier (area times density) to reach a figure. Herbert Jacobs, a journalism professor at Berkeley in the 1960s, derived a basic rule which has served as a guide since.
He worked out that a light crowd consists of one person per 10 square feet. A more densely packed gathering has an individual every 4.5 square feet, and a tight mob allows for no more than 2.5 square feet per person. ...
I'm sticking with the old method of a seasoned chronicler of protests: take the official estimate of the police, compare with the claim of the protesters, and go for somewhere in the middle.
That would put attendance at protests at somewhere between 3,500 and 6,000.
And the first comment at the Guardian:
I was there and I estimate that leaving Grant Park at the beginning of the march we covered an area at least as large as 34500 sq ft (1380 ft. x 25 ft).
We were densely packed and assuming the per person area was between 2.5 and 4.5 people per sq ft (per Jacobs), there were between 7666 or 13800 demonstrators at the beginning of the march.
So, that's the issue. Dense packing. Eye witnesses?