Only then did I notice that the far western end of Princes Street was blocked by police vehicles across all six lanes. I walked in that direction and realised that what I had seen was a very slow-moving march proceeding eastwards. Three police vans led the procession and the group of a few hundred marchers was completely surrounded by police officers - mainly from London. After around thirty minutes they reached the junction with the Mound. By this time several mounted police were lined up in Hanover Street and shortly afterwards a large steel barrier was erected across the road. This prevented more people from getting into Princes Street, but "normal" folk were being let out. I walked down Rose Street and re-entered Princes Street further west. It was possible to walk back towards the demonstration where several people had been arrested and were being processed and put into police vans. When I left, the demonstrators were surrounded by a ring of nose to tail police vans with some mounted police manoeuvring inside the "corral".
The Blogger system isn't allowing photos to be uploaded at the moment. They should be available later.