12 people per sec passing by. This was a steady stream, as it was reather nippy on the bridge and people just moved over it, without mmuch stopping. Hence I would say that this is a fairly safe estimate of the numbers of people attending the demo at this point of the march.
This would mean that in the one and a half hours it took everyone to cross Waterloo bridge 65.000 people crossed. I only took into account the people walking inside the Southbound lane, as occasionally (specially later on) many people also walked on the Northbound lane in the same direction then the rest of the demostrators - in fact mmmmost of these were demonstrators.