Video: 'Haymarket Strike'

(Tuesday, May 4, 1886, 10:30 p.m. local time) — A labor rally in Chicago calling for an eight-hour workday turned deadly tonight when a bomb exploded as police moved to disperse the gathering in Haymarket Square.

The demonstration began peacefully, a day after violence erupted at a rally outside the McCormick Harvesting Machine Co. plant on the city’s West Side. During that May 3 clash, two demonstrators were killed and numerous workers and police officers were injured.

Video: 'PBS: Haymarket Affair and the Origins of May Day'

At the Haymarket Square rally tonight, an unidentified person threw a dynamite bomb toward police as officers advanced on the crowd. The blast and the gunfire that followed left seven police officers dead and at least four civilians killed. Dozens more were wounded.

The episode, often referred to as the Haymarket affair, became one of the most significant and controversial events in the history of the American labor movement.