Ancient Greece consisted of several hundred more or less independent city-states (poleis). This was a situation unlike that in most other contemporary societies, which were either tribal or kingdoms ruling over relatively large territories.

Initially many Greek city-states seem to have been petty kingdoms; there was often a city official carrying some residual, ceremonial functions of the king (basileus), e.g. the archon basileus in Athens. However, by the Archaic period and the first historical consciousness, most had already become aristocratic oligarchies.  Through each stage more power would have been transferred to the aristocracy as a whole, and away from a single individual.

Inevitably, the domination of politics and concomitant aggregation of wealth by small groups of families was apt to cause social unrest in many poleis. In many cities a tyrant (not in the modern sense of repressive autocracies), would at some point seize control and govern according to their own will; often a populist agenda would help sustain them in power. In a system racked with class conflict, government by a 'strongman' was often the best solution.


Source: http://en.wikipedia.org/wiki/Ancient_Greece#Government_and_law