Scotland

As shit as facebook is, it's good at this kind of thing. A quick email to a local councillor will hook you up with litter pickers if you want I'm pretty sure. And a message to your local group(s) with a date, and people will join. A note in a shop window with the date also goes well too

Plenty of community councils in Scotland don't run, lack quorum, or are in effect secret. But everyone in the land should have this fourth tier of government