Two days later, on May 3, 1886, violence broke out at the McCormick Reaper Works between police and strikers when scabs were brought in to work in the Chicago plant. During speeches near to the McCormick factory, the police beat strikers with clubs and this situation escalated into rock throwing by the strikers. The police fired on the crowd and two strikers were killed. Numerous strikers were also wounded.
A public gathering was called for the next day in Haymarket Square to protest the police brutality. About 3,000 gathered in the rain to protest the outrage, including families and their children. The mayor of Chicago was present and testified later that the crowd remained calm and orderly.
As the gathering was nearing its end, the police began to move on horseback toward the wagon from which the speeches had been made. At this point, a person unknown, possibly an anarchist or a provocateur, threw a bomb into the massed police.
The police fired on the crowd. Seven or eight civilians were killed, and many times that were wounded by gunfire. Eight police in total were killed or died later from their wounds, primarily from their own fire.
Eight protest leaders, including three who were not even present in Haymarket Square on the fateful day, were tried for the “crime.” Five were sentenced to death and executed by hanging.