Well, that wasn't very nice.
On my cycle home tonight, I heard a large group of magpies and carrion crows making a real racket in a tree. It looked and sounded as if they were mobbing something, so I looked closer. Right at the top of the tree, there was another bird -- which appeared to be a small bird of prey, probably a kestrel -- hanging by its legs and flapping madly as the magpies tried to peck at it. It seemed as if the kestrel had something wrapped around its legs, which had snagged on a twig, and it couldn't free itself. It was right at the very top of a tree which I'd estimate was about 40m tall, so there was no chance that I could climb up to try to free it.
In the end, I called the RSPCA's emergency number, and after what seemed like ages giving my name, address and phone numbers, I managed to report what was happening. The problem was that this was a national call centre, and I was trying to describe a particular location in a park, so it was a bit frustrating. The operator said he didn't know if anyone would be able to come out, as it was "out of hours" (so much for an emergency line...), but I said I'd wait for half an hour or so anyway.
Watching an animal die a slow, painful death, when you are powerless to do anything to help is distressing to say the least. I watched it's frantic flapping get weaker and more intermittent, as the magpies got bolder at approaching and pecking the dying bird. Eventually, I couldn't stand it anymore, and had to leave.