I think there were about 10 of them - it looked like more were arriving. When I cycled through them they were totally oblivious to me. Whatever it was that had their attention, it was so consuming that there was no need to conceal weapons, there was no need to be aware of periphery human beings like me.
Nothing else happened but when I was safely out of the way I called 999. The person I spoke to said the call wasn't a waste of time. Five minutes later I was cycling back to Clapton via London Fields and I saw a police car driving through the park, then I noticed several police vans parked along the side of the park.
As I turned towards them to see what was going on I passed a group of three girls, I heard one of them mutter "nosey". I turned to look at her and she repeated it directly at me. I asked her if she knew what had happened. I don't think she answered.
I asked the police why they were there and they said nothing had happened... but that they'd got some call. I told them I'd called 999 earlier and asked if they thought it had been a waste of their time. They said that that was probably why they were there, but no, it hadn't been a waste of time.
Hopefully the police got more than one call - I can't believe I would have been the only person to have noticed something like this. Or may be it happens all the time. I saw something similar about a week earlier on Hackney Road, Asian kids on bikes (one with a baseball bat) obviously chasing a couple of black kids up towards the farm. I didn't call 999 then but felt as if I should have done.