Mexicans march over 1968 killings
The city's mayor said the country had a duty of gratitude to those who died
Thousands of people in Mexico City have paid tribute to those who died when security forces opened fire on a student demonstration 40 years ago.
The killings took place a few days before the Mexican capital hosted the 1968 Olympic Games.
The number of deaths in the incident is still disputed and no one has ever been jailed for involvement.
There were scuffles between police and demonstrators at the anniversary march, and about 20 people were arrested.
The government of the time said about 30 people died, but human rights groups and foreign journalists say about 300 were killed.
In the past decade, the Mexican authorities have set up several investigations into the deaths, but nobody has been jailed.
One protestor, Araceli Bernal, said she was attending the demonstration to "denounce the most despicable act committed in Mexico".
Speaking at the scene of the deaths, Mexico City's mayor, Marcelo Ebrard, said Mexican democracy was intact largely because of the people who died.
He said Mexico had a duty to be grateful to them.
The mayor said he supported attempts to uncover the truth about the events, and to get the Mexican government to accept responsibility for what happened.
The deaths in Tlatelolco Square took place during months of pro-democracy protests by students and workers in Mexico.
Security forces opened fire as thousands of people massed in the square. The government later said it had acted in accordance with the constitution to maintain order.
Human rights organisation Amnesty International has called on President Felipe Calderon to punish those responsible for what it called a massacre.
It said his failure to confront the clamp-down on thousands of students and workers had left a deep scar on Mexican society.