Defined sometimes as mobocracy. This is the rule of the mob. Very often not representative of the true majority, but very prominent in what they do. Disruptions and violence against legitimate proceedings.
I was amused to read in The Daily Beast, an article using this word. It accused Donald Trump and his supporters of ochlocracy. They included a history lesson going back about 1,500 years using the old latin definitions. Once again, to repeat what has been often said. This is that the Left, often accuses the opposition of exactly what they do themselves.