Tens of thousands of people marched in Rio de Janeiro yesterday evening to support Brazil's teachers in their fight for pay rises.

But the demonstration turned violent when a few hundred masked anarchists turned up at nightfall.

After several hours of peaceful protest, at least 200 "black block" anarchists smashed a city hall gate while others broke into banks, set cash machines alight and hurled firebombs.

Demonstrators also torched a bus on Rio Branco Avenue and pulled furniture out of banks to use in barricades as they squared off with police who responded with tear gas.

At first only unarmed security guards were on hand to stop the town hall break-in but riot police eventually showed up.

Afterwards the streets looked like a battle zone, with buildings damaged, trash burning and the smell of tear gas.

A similar rally in Sao Paulo also turned violent. Seven people were injured, including four police officers.

Teaching unions have been pressing for a pay raise for two months, saying that if the country can afford to host huge sporting events such as the 2014 World Cup and 2016 Olympic Games, it can certainly afford a wage rise for their hard-pressed members.

They said that 50,000 people had marched to support them before the violence broke out.