The protesters called for the 43 students from a rural teachers' college in Guerrero state, missing since September 26, to be returned alive. Fears have grown that 10 newly discovered mass graves could contain their bodies.
Attorney general Jesus Murillo Karam said the motive behind the apparent massacre of the student teachers, who went missing after clashing with police in Iguala in the volatile, gang-ridden state of Guerrero, was not yet clear.
Guerrero's attorney general, Inaky Blanco, said 28 bodies had been found at a mass grave site so far, adding it was "probable" some of the missing 43 students are among the remains.
October 13, 2014: A sandal lies in an alleged drug camp where at least six more mass graves were foundAFPOctober 13, 2014: A billboard offering a million Mexican pesos ($74,000) as a reward for information leading to the whereabouts of 43 Ayotzinapa Teacher Training College students is seen on the outskirts of IgualaReuters
The violence came more than two weeks after police in Iguala, also in Guerrero state, opened fire on the teacher's college students, killing at least six.
The students, from Isidro Burgos College, went missing after they clashed with police, who shot at a bus that they claim had been hijacked by the youths.
Witnesses said dozens of students were taken away by police and have not been seen since. Twenty-six local police officers have been detained and officials are attempting to determine if any of the students are in the mass graves nearby.
September 29, 2014: Student Andres Martinez lies in the General Hospital in Iguala after being injured during clashes that led to at least six deaths and the disappearance of many studentsAFPSeptember 29, 2014: A statue of Jose Maria Morelos y Pavon, a Mexican revolutionary rebel leader, is seen after it was toppled during a riot by Ayotzinapa Teacher Training College students, at the entrance of the City Congress building in Chilpancingo. Students from the college demonstrated in the streets to demand the safe return of the missing studentsReutersSeptember 30, 2014: People gather around the coffin during the funeral of Julio Cesar Ramirez, one of six killed during clashes that resulted in the disappearance of many studentsReutersOctober 2, 2014: A man wearing a hood with crosses on it takes part in a demonstration to protest against the disappearance of 43 students from Ayotzinapa Teacher Training CollegeReutersOctober 2, 2014: Students wearing Guy Fawkes masks take part in a demonstration in Mexico City to demand authorities find the 43 missing studentsAFPOctober 3, 2014: Ayotzinapa Teacher Training College students hold flaming torches during a demonstration in the streets of ChilpancingoReutersOctober 4, 2014: A journalist takes a picture of a car in the driveway of Guerrero Governor Angel Aguirre's residence, overturned by protesters from the Ayotzinapa Teacher Training CollegeReutersOctober 7, 2014: Clandestine graves are seen at Pueblo Viejo, in the outskirts of IgualaReutersOctober 8, 2014: People gather outside Guadalajara Cathedral in support of the missing students from Ayotzinapa Teacher Training CollegeReutersOctober 8, 2014: Photos of missing students are seen during a march in Mexico City in support of the missing studentsReutersOctober 8, 2014: A member of a vigilante group enters a house in Las Juntas community in Colula, Guerrero state, while looking for the missing studentsAFPOctober 12, 2014: A federal policeman takes pictures of a banner hung on a wall in IgualaReutersOctober 12, 2014: Iguala residents and students from Ayotzinapa Teacher Training College demand the return of 43 missing studentsReuters
The confrontation in Iguala shed light on a widespread problem with local police in Mexico: they are often allegedly linked to organised crime. In the case of Iguala, the police who attacked the students were reportedly working with the local cartel, Guerreros Unidos, according to testimony of those arrested.