Riot police have clashed with protesters in Mexico City during a demonstration commemorating the 45th anniversary of a student massacre.
Protesters, some of them masked, threw firebombs, bottles and rocks at police who battled to disperse the crowd.
At least 40 people were injured, Mexico’s El Universal newspaper reported.
The rally marked the anniversary of the 1968 killings of student protesters in Tlatelolco Square.
Official reports at the time said 25 people died, although rights activists say as many as 350 may have been killed.
The anniversary of the killings is often marked by protests and violence.
In the latest clashes, police said 20 officers had been injured and 20 people were arrested. Authorities said anarchists had infiltrated the march.
Earlier, thousands of teachers and students blocked the city’s main roads during rush hour to honour victims of the massacre.
The deaths in Tlatelolco Square took place during months of pro-democracy protests by students and workers.
The killings took place a few days before the Mexican capital hosted the 1968 Olympic Games.