The Haymarket Riot or Haymarket Massacre took place in 1886 in Haymarket Square in Chicago, during a demonstration for 8 hour shifts, which also a protest against the killing of several workers by the police the day before. Someone threw a bomb to the police, which blasted killing some police officers and civilians and hurting many others.
Eight anarchists were convicted for the bombings. Five of them got a death sentence. Years later it would be proved that the judicial process was illegitimate and the convicted became the Chicago Martyrs.
These riots were very relevant because they were the origin of the International Workers' Day, that is celebrated worldwide on the 1st of May.