Tlatelolco massacre

After a summer with large protests against the 1968 Summer Olympics held in Mexico City, the Mexican Armed Forces shot multiple unarmed civilians, killing an unknown number in the hundreds. The shooting happened in the Plaza de las Tres Culturas in the Tlatelolco section of Mexico City, and is considered part of the Mexican Dirty War. The massacre happened ten days before the opening ceremony of the Olympics.
The head of the Federal Directorate of Security reported that 1,345 people were arrested. At the time, the Mexican government and media claimed that the Armed Forces had been provoked by protesters shooting at them. However in government documents released in 2000, it said that snipers had been hired by the government. The actual death toll range from 300 to 400, with eyewitnesses reporting hundreds dead.