Estimating crowd sizes is difficult even if you don't have good visibility, and for most events, there are at least two or three sets of people estimating crowd size who have axes to grind that bias their results. Washington DC's especially bad about that.
According to the newsblurb we heard in San Francisco, the Washington DC park police have stopped routinely giving out estimates of crowd size, but it was in the "tens of thousands", an number that stretches from about 10,000 to 1,000,000. Their estimates were often politically driven, and I don't know if their refusal to estimate is to avoid getting caught in politics (probably) or because it sounds smaller than the probable size so it's still political. I saw some CSPAN coverage of the speakers later that night, but didn't see any wide-area crowd shots, just a few near-stage ones. The San Francisco event did pack the square in front of city hall, and apparently during the march from Justin Herman Plaza to city hall, they pretty much filled the street all the way - it's about 1.5 miles. I don't know how many people it takes to fill the plaza, but 42,000 is a believable number. If you were hearing it consistently, that means that one source reported it first and everybody else copied it (probably either the police or the organizers.) We were trying to meet some people who were on Market Street, so we didn't get to see the crowd on Justin Herman plaza, and crowd dynamics made it impractical to stand on anything tall enough to see crowd size when things were getting started. The crowd stayed relatively compact until the march started - while we were looking for parking on the way there, we were hoping that it would be larger than the crowd farther down the Embarcadero who were there for the Red Bull FlugTag fair, but once we got to the main area, no worries about that :-)
