Violent clashes between police and members of a teachers' union in Mexico have left six people dead and more than 100 injured.

Activists from the National Co-ordinator of Education Workers are opposed to the mandatory testing of teachers as part of education reform.

They are also protesting over the arrest of union leaders on money laundering and other charges.

In clashes in the southern state of Oaxaca, protesters threw stones and petrol bombs, and burned vehicles, while riot police were said to have fired on protesters.

Police said all the dead were civilians with two having ties to the union.

Earlier, Mexico's government released a statement saying 21 federal police had been wounded, three of them by gunfire, and that its agents who participated in the operation were not carrying guns.

Footage showed at least one police officer firing a gun several times, though it was unclear if he was a federal or state agent.

Ten years ago, the teachers started a six-month takeover of Oaxaca that did not end until police stormed the barricades.