>>16888 TL;DR: Protests are occurring over the arrest of an anti Putin critic called Alexei Navalny. ОМОН (Отряд Мобильный Особого Назначения, previously Отряд Милиции Особого Назначения, Otryad Mobil'nyy Osobogo Naznacheniya, Special Purpose Mobile Unit) got sent to stop and arrest everyone they could get. The Roskomnadzor (literally the Russian internet police) is monitoring everyone that attends or talks about the protests and is arresting them en masse.
Here's the url to the original thread post that I forgot to include here >>16885