How many there?
A predictable feature of any demonstration is a large gap between the numbers said to be present by the Police and by the protest organisers.
Take for example the Stop the War demo in London in February. The organisers spotted six times as many marchers as the Police did - they said 60,000, whereas the Police gave out a figure of 10,000.
Estimating the size of a large, scattered, varying, unregimented and mobile crowd is a far from easy task. But a factor of six is a pretty big discrepancy too.
The BBC put a freedom of information request to the Metropolitan Police for full details of the process used to calculate the numbers present at this demonstration.
This is how they replied (incidentally after about six months rather than within the legal deadline of 20 working days):
"The GLA Licence Trafalgar Square for 19,999 persons. This, they estimate, is Trafalgar Squares capacity. The North Terrace Capacity is approx. 8,000, with the remainder of the Square being approx. 12,000. So, on the 24th Feb, with the majority of the North Terrace being barriered off and with Staging & Screens on the main Square, capacity is reduced. On this occasion numbers were estimated by an officer standing on the steps of the National Gallery during the event. After taking into consideration of the above, and the large gaps around the Square, they estimated the numbers never exceeded 12,000 at any one time.'
However, a maximum figure at any one time doesn't tell you how many people took part in the protest at some point, as marchers came and went through Trafalgar Square. This would suggest that the Police figure was certainly an underestimate.