Vale condemned the action in a statement, saying it "repudiates violence and the constant threats to its patrimony and life of its workers by the MST".
Land reforms
On Saturday, MST activists invaded a Vale-owned forestry and charcoal unit near the company's plant in Carajas - the Amazon area where Vale's biggest iron ore mine is located.
Last October the group, along with Via Campesina, also blocked a railroad near the Carajas plant for two days, briefly leaving the plant without raw materials.
In Brazil, land considered unproductive can be expropriated by the government for redistribution to the country's poor.
About 3.5 per cent of landowners hold 56 per cent of arable land in Brazil, while the poorest 40 per cent own only one per cent.
In recent years the groups have also turned their attention to targeting large companies and multi-nationals, invading land they consider unproductive in order to pressure the government.