As someone who used to organise large demonstrations in London for CND [a UK anti-nuclear org] a long time ago -- there is
no point that the crowd is
ever so large that a combination of the organasition itself with the police where the crowd "should" be unable to control it. In fact when you're putting together a demo in London you meet the Met police and meeting that requirement is one of the things you discuss with them.
On Feb 15 2003 a demo against the impending second war in Iraq was so ridiculously larger than anyone (police and organisers included - it was at least a million people) expected that I personally [long story] caused Waterloo Bridge to be closed at about 2pm. The senior police on the day were so understaffed relative to the crowd size that not only did I close a major traffic artery but we volunteers with some previous experience also got called out by the Metropolitan Police on TV for thanks.
Still was a very strange feeling to, after years earlier spending many similar days being condescended at by untrained idiot coppers, suddenly being drafted by their bosses into making major crowd-control strategic decisions when the situation had become incredibly dangerous mostly because those same police had utterly misgauged the turnout().
( On that day having left the org in question some years previously I didn't believe it was going to be that big either. And had turned down their request to help. Then I was in a bank in town a few days before surrounded by ordinary regular nonpolitical people and everybody in the queue joined in a discussion about which tube station they needed to be at for Saturday. Both I and the org were wrong about how big it was going to be by a factor of between 2 and 5.)