According to the New York Times*, there’s lately been a rash of single-person protests in Moscow.
Demonstrations require authorization, which often isn’t given. A single person, though doesn’t count as a demonstration, and so can stand anywhere they like, holding a placard with impunity.
Or almost with impunity. Counter-protesters (in this case, government supporters) have a kamikaze option. They join the protest, with a placard giving the opposite view. Now it’s an illicit two-person demonstration, and all participants can be arrested:
Under a quirk of Russian law on rallies and protests, so-called individual pickets are legal without permits, which the opposition rarely obtains. Single protesters, standing 30 feet or so apart, may hold signs in public. As the Russian police were interpreting the rules, two protesters standing together were grounds for arrest — even if they came from opposite sides of the political spectrum.
* all the reports I’ve found on this lead back to the New York Times or Washington Post; I’ve not found anything on Russian blogs, etc. I don’t think it’s been made up, but I’m not going to spend long checking.