Thousands of teachers and students have blocked Mexico City's main thoroughfares during rush hour to honour victims of a 1968 massacre while pledging to fight to undo an overhaul of the education system.
A small group of self-described anarchists clashed with riot police, who used tear gas after the activists threw firebombs, bottles and rocks.
Twenty officers were injured, and 20 people were detained.
Yesterday's protests and road closures were just the latest acts in what has become near daily disruption by demonstrations of the capital's life in recent weeks.
The education reform aims to break union control of the school system by requiring teacher evaluations.
The anniversary of the 1968 massacre of students by soldiers is often marked by protests and clashes.