Clashes between Mexican riot police and protesters over the case of a number of missing students have left at least 11 police officers injured in Mexico's southern state of Guerrero.

On Tuesday, police blocked buses carrying relatives of the 43 young men and fellow students who were heading from the town of Tixtla to the state capital, Chilpancingo, to hold a demonstration.

Protesters threw Molotov cocktails at some 200 police officers, who responded with tear gas.

Civil protection officials said the scuffles left at least 11 officers wounded. Two students were also hurt, according to the protesters.

Masked students set fire to a lorry that they had already seized. 

The scuffles came a day after a group of protesters ransacked the prosecutor's office in Chilpancingo, breaking furniture and computers.

The confrontation took place in the run-up to the first anniversary of the disappearance of the students. A protest is planned for Mexico City on Saturday to mark one year since the 43 students disappeared.

On September 26, 2014, the 43 students, all trainee teachers, disappeared in the city of Iguala in Guerrero state, following an attack by police forces suspected of having links to drug gangs. The incident took place during a protest rally over teachers’ rights.