Thousands of teachers and students blocked Mexico City's main streets on Wednesday to commemorate victims of a 1968 massacre while pledging to fight to undo government changes to the education system.

Riot police clashed with the protesters, leaving dozens injured.

The Red Cross said that at least 51 people were injured when police used tear gas against a group of masked protesters throwing rocks and firebombs.

Police estimated that 32 officers had been injured and said they had detained at least 97 people.

Dozens of protesters broke the windows of shops, bus stops and a national newspaper office.

Police officers used shields to defend themselves and charged at protesters and journalists alike.

At least 10 journalists were attacked by police and by some protesters, according to press rights group Articulo 19.

The demonstrators demanded justice for killings that took place on October 2 1968 when soldiers opened fire on 8,000 protesters in the capital, leaving an official death toll of 40. Many student groups however say that as many as 300 people were killed.

This year, the demonstrators were joined by striking teachers who have been protesting in Mexico City since August against education reforms aimed at breaking the power of teaching unions.

Demonstrations are held every year to mark the massacre in the Tlatelolco neighbourhood.

Wednesday's march was the first since the ruling Institutional Revolutionary Party (PRI) returned to government after a 12-year absence.

The violent repression of 1968 occurred in the middle of a long PRI reign.

The party held power for 71 years through a mix of rigged elections, corruption and patronage until it was voted out in 2000.

President Enrique Pena Nieto, who took office in December, has claimed that the PRI now embraces democracy.

But protesters criticised the PRI and Mr Pena Nieto, demanding that those responsible for the massacre must face justice.

The president at the time, Gustavo Diaz Ordaz, later accepted responsibility for the repression but died in 1979 without facing justice.

"There must be justice and reparations," said Emilio Reza, a member of the 68 Committee of survivors and their relatives.