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Run by the people behind 'No Trousers on the Tube', this event is a version of Improv Everywhere's MP3 Experiment, which has previously been run in the States (this is their most recent effort: https://youtu.be/nIwzorwYTqs).
Download the MP3 before the event, then meet at the pagoda outside the Chinese Bakery before 3pm, with apparatus to play it on. After a countdown, an airhorn will sound on the dot of 3pm: press play.
Beyond that... we don't know. The organisers will be wearing red baseball caps and handing out 'props' (scary scare quotes, those!) but promise you won't have to do 'anything too silly, but you'll have a lot of fun and also see parts of London you likely hadn't seen before'.
Note: over-15s only. You only have to turn up to take part, but the organisers would appreciate it if you sign in on the Facebook page to give them a rough idea of numbers.