The Collaborative International Dictionary of English v.0.48:

Mobocracy \Mob*oc"ra*cy\, n. [Mob rabble + -cracy, as in
   democracy.]
   A condition in which the lower classes of a nation control
   public affairs without respect to law, precedents, or vested
   rights.
   [1913 Webster]

         It is good name that Dr. Stevens has given to our
         present situation (for one can not call it a
         government), a mobocracy.                --Walpole.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mobocracy
    n 1: a political system in which a mob is the source of control;
         government by the masses [syn: mobocracy, ochlocracy]