The world's first democratic constitution was put into effect by

Athens

Explanation:

Athenian democracy developed in the Greek city-state of Athens comprising the central city-state of Athens and the surrounding territory of Attica around 550 BC. Athens is one of the first known democracies. Other Greek cities set up democracies, and even though most followed an Athenian model, none were as powerful, stable, nor as well-documented as that of Athens.