Let's put some perspective on this. The police said "200,000" went to London to protest the war - whereas unofficial figures reckoned a million people (in short, a lot higher than the police). A million people is a serious, serious fuckton of people and anyone who's been to Wembley will surely agree that a million marching simply doesn't seem realistic. It's not likely the Police would get it too far wrong, given that there are lots of police around to count the crowds, and they will have to use the crowd numbers to judge the number of officers required for each section of the route.
Funnily enough, that's how many they apparently got at the Treble parade, although they all seemed to be following behind the bus, so they probably counted them all a few times over for good measure.
But let's give them the benefit of the doubt, and say the police might have overestimated. The route wasn't all totally heaving, and if there were 12,000 in Albert Square, I'm guessing most dispersed from there to catch up with the bus elsewhere on the route, so let's be generous and not count those.
1.8miles route = approx 3000yds, with all pavements at least 3yds wide, so let's be sensible and say you get 9 people in every sq yard of pavement, so that's 3000 x 3 x 9 = 81,000. PLUS those rammed into the side streets. PLUS those who didn't rejoin the route from Albert Square and just went home again.
Even being generous, I fail to see how anything less than 90,000 would be a remotely sensible guess.
And as if there was a single rag in town yesterday to corroborate or deny the police figures anyway!!