Mexican police have fired at a bus seized by 60 students from the Ricardo Flores Magon Student Committee of Ayotzinapa.
Twelve students were arrested in the ensuing clashes.
The students had been raising money in support of the 43 students from Ayotzinapa teachers' training college who went missing in 2014
Families of the 43 disappeared have denounced the police aggression and called for an investigation into the officers.
"We condemn the artery aggression against our youth and demand an end to the repression and criminalization of Ayotzinapa, the use of excessive force against our comrades, as well as the use of weapons in public demonstrations," said the parents.
The Ayotzinapa Student Committee said the incident will not deter them from their cause.
"If raising our voice and demanding truth and justice will (mean continued aggression) we will take the risk. We have 43 reasons to do it,” the committee said in a press release.
Earlier on Wednesday in Guerrero, students seized a tanker truck with 30,000 liters of fuel and several other vehicles to demand justice for the 43 disappeared.
Ten students were arrested in the protest.
Four policemen were later captured by the students. They were then released after the detained students were freed
In July, a bus carrying members of the Nochixtlan Victims Committee for Truth and Justice was shot at on its way to an Ayotzinapa demonstration.
Around 40 passengers were on board when masked men stopped the bus just after midnight on the Oaxaca-Mexico highway and began to shoot at the vehicle.