The Haymarket incident was a major incident in the history of US labor unions. The incident occurred on May 4, 1886.
The Haymarket incident took place as the result of strikes that had been going on. The day before the incident, two strikers had been killed by police. The gathering at Haymarket Square was a rally in protest of these killings. At some point in the rally, the police tried to break the crowd up. When they did, someone threw a bomb into their ranks. Seven policemen were killed and about 60 were wounded. In the aftermath, the police fired into the crowd. It is not know exactly how many protestors died, but estimates are that both the dead and wounded among them were roughly equal to the number of police casualties.
Seven people were convicted of murder, not for actually throwing the bomb, but for their words, which were said to have incited the murders. Four were executed, one committed suicide and the others were eventually pardoned. The incident is credited with destroying the Knights of Labor, the first major national labor union in the US.