Protesters demanding justice for 43 students who disappeared nearly a decade ago launched fireworks at police in Mexico on Monday, leaving at least 26 officers injured, authorities said.
The police were treated by paramedics and taken to hospital after the demonstration outside the presidential palace in Mexico City, the capital's security secretariat said.
The protest came after eight soldiers accused of involvement in the disappearance of the students from a teacher training college in the southern state of Guerrero in 2014 were freed from pre-trial detention.
The case is widely considered to be one of the country's worst human rights atrocities.
In March, protesters angered by a lack of progress in bringing the perpetrators to justice smashed open a door to the presidential palace.
The students had been traveling to a demonstration in Mexico City when investigators believe they were kidnapped by a drug cartel in collusion with corrupt police.
The exact circumstances of their disappearance are still unknown, but a truth commission set up by the government has branded the case a "state crime," saying the military shared responsibility, either directly or through negligence.
Arrests have been made or ordered for dozens of suspects, including military personnel and a former attorney general who led a controversial investigation into the mass disappearance.
So far, the remains of only three of the victims have been identified.