The Thirty Tyrants were a pro-Sparta

History of Sparta

The History of Sparta describes the destiny of the ancient Dorian Greek state known as Sparta from its beginning in the legendary period to its forced incorporation into the Achaean League under the late Roman Republic, its conquerors, in 146 BCE, a period of roughly 1000 years...

n oligarchy

Oligarchy

Oligarchy is a form of power structure in which power effectively rests with an elite class distinguished by royalty, wealth, family ties, commercial, and/or military legitimacy...

 installed in Athens

Classical Athens

The city of Athens during the classical period of Ancient Greece was a notable polis of Attica, Greece, leading the Delian League in the Peloponnesian War against Sparta and the Peloponnesian League. Athenian democracy was established in 508 BC under Cleisthenes following the tyranny of Hippias...

 after its defeat in the Peloponnesian War

Peloponnesian War

The Peloponnesian War, 431 to 404 BC, was an ancient Greek war fought by Athens and its empire against the Peloponnesian League led by Sparta. Historians have traditionally divided the war into three phases...

 in 404 BC

404 BC

Year 404 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Tribunate of Volusus, Cossus, Fidenas, Ambustus, Maluginensis and Rutilus...

. Contemporary Athenians referred to them simply as "the oligarchy" or "the Thirty" ; the expression "Thirty Tyrants" is due to later historians. Its two leading members were Critias

Critias

Critias , born in Athens, son of Callaeschrus, was an uncle of Plato, and a leading member of the Thirty Tyrants, and one of the most violent. He was an associate of Socrates, a fact that did not endear Socrates to the Athenian public. He was noted in his day for his tragedies, elegies and prose...

 and Theramenes

Theramenes

Theramenes was an Athenian statesman, prominent in the final decade of the Peloponnesian War. He was particularly active during the two periods of oligarchic government at Athens, as well as in the trial of the generals who had commanded at Arginusae in 406 BC...

.
The Thirty severely reduced the rights of Athenian citizens. Imposing a limit on the number of citizens allowed to vote (limiting the franchise for example to the wealthiest citizens) was a standard move on the part of wealthy people who objected to being subject to the votes of the "rabble" in a broad-based democracy where all free adult males could vote.