@budududuroiu @Geri That's a good method! However, the people in the picture on the left are largely confined to a road, which stretches out of frame on both sides; you'd also want to take into account how much of the road is occupied when making your estimate. Is there any way to get an idea of that?
But yeah: the Together Alliance want their (reported) crowd sizes to be bigger than the (reported) crowd sizes in the far-right rallies, so there's motive. (The Met also has motive.)