Veterans of Mexico's 1968 student movement and younger generations of activists marched in Mexico City to remember the day 50 years ago when army troops massacred hundreds of people at a peaceful protest.
Five decades on, it remains unclear exactly how many people were killed when the army opened fire on a rally of about 8,000 students in the Plaza of Three Cultures, in central Mexico City's Tlatelolco neighborhood.
The authoritarian government at the time put the death toll at 20, claiming "terrorists" attacked the troops, forcing them to defend themselves.
Independent reports say anywhere from 300 to 500 people were killed.
The massacre -- which came just 10 days before Mexico City hosted the 1968 Olympics -- remains one of the darkest episodes in a year of global turbulence.
The government's failure to bring those responsible to justice or explain what happened that day has become a rallying cry for the Mexican left and provokes annual protests that often turn violent.
There was a heavy police presence for Tuesday's march in central Mexico City, as thousands of people marched from the plaza where the massacre occurred to the capital's central square, the Zocalo.
They carried signs with messages such as "They keep killing us" and "The state kills and disappears students."