Apparently there was a protest against Trident yesterday in Trafalgar Square dubbed "Bin the Bomb". As with all protests the estimate figures always make for amusing reading. According to the Stop the War coalition, CND, the British Muslim Initiative, and Respect, there were 100,000 people at the protest. In comparison the Metropolitan Police estimated that between 2,000 and 3,000 took part.
Now the Police and protest organisers always have wildly different views of how many people attend protests. The organisers big themselves up, whilst the Police play them down. The thing is, it's rare that the figures are so massively different. Of course, over counting your level of support is probably not difficult in Trafalgar Square on a Saturday.
It's likely that many tourists have suddenly found themselves counted as members of the extremist left/homophobic Islamist coalition.