There are many examples of unneccesary stop-and-search under the terrorism acts but, as Dave Hill tells us: "John Yates, Assistant Commissioner Specialist Operations, has today reminded all MPS officers and staff that people taking photographs in public should not be stopped and searched unless there is a valid reason."
"I'm a Photographer, Not a Terrorist! invite all Photographers to a mass photo gathering in defence of street photography"
I hope the fact that this evening, just now in fact, the UK terror threat level was being raised from "substantial" to "severe" has nothing to do with this i.e. an excuse to close down/stifle the demonstration.
Update: Saturday pm. Quite a good turn out by all accounts [Link]. The guy in the yellow jacket looks familiar ;-) OH... and they even had it on the evening news here. Article in Spanish HERE, (8:30pm).
The new alert level means a terrorist attack is considered "highly likely".