Haymarket Riot, a bombing and shooting incident that occurred when police tried to break up a gathering of striking workmen on May 4, 1886, near Haymarket Square in Chicago. A bomb thrown into police ranks killed 7 policemen and injured 70 more. The police then fired into the crowd, killing or wounding at least 50 workmen. The street meeting had been called by August Spies, editor of an anarchist newspaper, to protest the May 3rd shooting at the McCormick Harvesting Machine Company of several workers who had been striking for an eight-hour work day.

The identity of the person who threw the bomb was never established. Nevertheless, eight prominent anarchists were arrested and convicted of conspiracy to commit murder. Four, including Spies, were hanged, and one committed suicide. In 1893 the three remaining prisoners were pardoned by Illinois governor John P. Altgeld, who called the trial a miscarriage of justice. The Haymarket incident hurt the labor movement by associating it in public opinion with violence and revolution.