Landless activists blocked a key export railway of CVRD.
It was the third time in less than a month that activists from the Landless Workers Movement, or MST, invaded the railway to block CVRD's shipments, the company and authorities said.
Companhia Vale do Rio Doce SA, the world's largest iron ore miner, uses the railway at the Carajas mining complex to transport 250,000 metric tons (275,600 tons) of iron ore daily from its sprawling Amazon mine to Atlantic ports for shipment overseas.
The company said the blockade is also keeping 1,300 local passengers from using the railway and stalling fuel shipments to small cities in rural Para state.
CRVD said in a statement that about 300 MST members - some of them armed - surrounded two trains near the northern city of Paraupebas and cut brake lines. It said four workers and a train operator were held for about half an hour, then released.
Para state police officer Jose Monteiro said by telephone that the protesters had refused to allow repairs to the trains, but there were no reports of injuries.
CVRD has invested heavily to boost production at its Carajas mining complex, but the railway that carries its ore to the Atlantic has been periodically shut down by protesters seeking concessions from CVRD and Brazil's government.
CRVD's U.S. traded shares were down 2.4 percent at US$36.87 (25.04 EUR) on the New York Stock Exchange on Wednesday.
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