It's quite sad that the Mexico Summer Olympics is perpetually associated with this terrible event, due to the proximity of the occurences. The Tlatelolco Massacre, a.k.a. The Night Of Tlatelolco had taken places on the afternoon to night of the 2nd of October, 1968 in Plaza de las Tres Culturas in the Tlatelolco section of Mexico City.
What exactly happened here? It begins with students from all around Mexico taking advantage of the Olympics spotlight and rallying for specific protests about the current government. Some of its protests included the following: repeal of Articles 145 and 145b of the Penal Code (that sanctioned imprisonment of anyone attending meetings of three or more people deemed to threaten public order).; freedom of politcal prisoners, abolition of the granaderos (tactical police corps) and the dismissal of the chief of police and his deputy.
The protests steadily grew in number and exposure and the government had failed to prevent it. Eventually, President Gustavio Diaz Ordaz ordered a military take-over of the biggest university in Mexico-- the National Autonomous University. On October 2, approximately five thousand people-- mostly students and their families joined the protests at this same place, this time, protesting the presence of the militia at their school. At sunset, increased military units were deployed to surround the area, and the carnage began, as rounds after rounds of live ammunition were fired at the helpless protesters.
The death toll of the massacre was atrocious and one of the most controversial figures in Olympic, with most sources reporting 200 to 300 deaths. The government being the government reported only 4 dead students and 20 wounded, a gross and perverse repression of a serious case of genocide. Aside for the mortality statistics, there was also much controversy regarding the exact number of people arrested fr the cause.
All this happened because of the politicalization of the Olympic games, a trend that has occurred quite recently in the upcoming Beijing Olympics.