The Tlatelolco Massacre

Some ten thousand people were at a peaceful rally in the square when the army sealed the exits and firing began from the surrounding buildings. The government said students had shot first; it emerged decades later that a presidential guard unit on the rooftops had opened fire to provoke exactly that account.

Official figures gave a few dozen dead. Independent estimates run to several hundred, and the true number has never been established. A stele in the square carries the names that are known.