How Many People Were In Trafalgar Square On Saturday?

Simple enough question, I should have thought, but as usual the police and STWC organisers are vastly at odds.  Let's throw some facts on the table:

Police, according to BBC:

While the same source on the STWC:

Which differs from STWC's website:

Meanwhile the Met, having cranked up the random number generator, told the Independent (and Observer):

At Lenin's Tomb the man himself claims (in comments)

Rachel, who was there, says in her comments:

Whlle Davide's report on Nether-World estimates

All of which proves not a lot, except that it's extremely unwise to trust the police figures, which in statistical terms go in the discard pile. Let's expand the data set a bit - the Square isn't used solely for anti-war marches, and there have been two other large crowd events in the last couple of years that received wide coverage - the first is the 6/7/2005 2012 Olympics Award:

 The second is the Ashes celebration in September 2005.

[Note the photos, particularly of Michael Vaughan in front of a frankly enormous crowd]

and additionally

 while the Scotsman has

Now, the Square is 110x110 metres, according to the GLA.  That's 12100 square metres.  Now, I reckon you can get four people in per square metre, which brings us up to a notional maximum capacity of, broadly, 50,000.  10,000 people would make the square look decidedly empty - if your commuter train came into the station 20% full you'd be well happy.  From the photos I can see, the area is not 20% full, it's more like 80%, which would put 40,000 people in the square alone when Davide was taking his photos, not counting those still on the march.

So, what have we established?  A large turnout for the Square itself is 30,000 to 50,000 people.  More people were outside the square.  I reckon we can conservatively estimate the turnout to be 50,000+, or sixteen times what the Met Police told the country's leading non-conservative Sunday broadsheets.  Mind you, since their own website of forthcoming events doesn't mention the 24th February demo, perhaps they forgot.