The order to close the General Artigas Bridge linking the two countries across the Uruguay River was given by Uruguay's President Tabare Vazquez after Argentine protestors said they would head near the site of the mill, naval coast guard sources told AFP.

Environmentalists and residents in the Argentine border area fear that the plant, owned by the Finnish firm Botnia in the nearby Uruguayan town of Fray Bentos, will pollute the river that runs along the border.

Protestors in the Argentine town of Gualeguaychu have relentlessly kept up another roadblock at the border for the past year.

The latest closure came into force at 6:00 pm (2000 GMT).

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