Plutarch wrote:

Solon made it a law that anyone who refused to take sides in a revolution would lose all civil rights. By this law he made sure that the good would resist the bad and not hide hoping to save themselves, or wait until they could see which side will win.

Perhaps on the basis of this passage from Plutarch's biography of Solon, it has been said that all Greeks were required to vote in ancient Athens.

"In ancient Greece, the inventors of democracy set up a law that required every person to vote, regardless of who they voted for. If anyone was found not voting, the person would be publicly marked and labeled an idiot, someone who thought their own personal needs trumped those of the society around them, and over time, the word "idiot" has evolved into today's usage."
Isaac DeVille, Michgan State columnist

Find out why this isn't the case, in Were All Ancient Greeks Required to Vote or Risk Being Labeled Idiots?
Plutarch Picture © Clipart.com