Mexican President Andres Manuel Lopez Obrador urged protesters to rally peacefully on Monday's eighth anniversary of the disappearance of 43 students -- one of the country's worst human rights tragedies.
The appeal came after several dozen members of the security forces were injured on Friday in clashes with protesters hurling rocks and homemade explosive devices at a Mexican military base.
"The only thing is to avoid violence," Lopez Obrador told reporters hours before a planned march in Mexico City, declaring Monday a day of national mourning.
"It's not about throwing stones or Molotov cocktails. It's about protesting peacefully," he said.
The 43 teaching students had commandeered buses in the southern state of Guerrero to travel to a demonstration in Mexico City before they went missing.
Investigators say they were detained by corrupt police and handed over to a drug cartel that mistook them for members of a rival gang, but exactly what happened to them is disputed.
So far, the remains of only three victims have been identified.
Last month, a truth commission tasked by Lopez Obrador's government to investigate the atrocity branded the case a "state crime" involving agents of various institutions.
It said that military personnel bore "clear responsibility," either directly or through negligence.
Arrest warrants have been issued for more than 80 suspects, including military personnel, police officers and cartel members.
Former attorney general Jesus Murillo Karam, who led a controversial investigation into the mass disappearance, was detained last month on charges of forced disappearance, torture and obstruction of justice.
According to an official report presented in 2015 by the government of then-president Enrique Pena Nieto, cartel members killed the students and incinerated their remains at a garbage dump.
Those conclusions were rejected by relatives, independent experts and the office of the United Nations High Commissioner for Human Rights.