Haymarket affair: In Chicago, United States, a homemade bomb is thrown at police officers trying to break up a labor rally, killing one officer. Ensuing gunfire leads to the deaths of a further seven officers and four civilians.

The Haymarket affair, also known as the Haymarket massacre, the Haymarket riot, the Haymarket Square riot, or the Haymarket Incident, was the aftermath of a bombing that took place at a labor demonstration on May 4, 1886, at Haymarket Square in Chicago. The rally began peacefully in support of workers striking for an eight-hour work day; it was held the day after a May 3 rally at a McCormick Harvesting Machine Company plant on the West Side of Chicago, during which two demonstrators had been killed and many demonstrators and police had been injured. At the Haymarket Square rally on May 4, an unknown person threw a dynamite bomb at the police as they acted to disperse the meeting, and the bomb blast and ensuing retaliatory gunfire by the police caused the deaths of seven police officers and at least four civilians; dozens of others were wounded.