Government by a small group of all powerful persons :

One word substitute is Oligarchy


Oligarchy: a small group of people having control of a country or organization.
Monarchy: a form of government with a monarch at the head.
Democracy: a system of government by the whole population or all the eligible members of a state, typically through elected representatives.
Anarchy: a state of disorder due to absence or non-recognition of authority or other controlling systems.