Hundreds of Argentineans crossed into Uruguay amid tight security to protest outside a paper pulp mill, the cause of a major dispute between the countries.
Argentina and environmentalists say the plant pollutes rivers. Uruguay and the mill's Finnish owners say it will not.
Protests have been held on Argentina's side of the river for almost two years but this is the first time they have crossed in such numbers to Uruguay.
No one appears to have a solution to the long-running conflict.
The demonstrators started going through customs controls on the long bridge that links the two countries first thing in the morning.
They complained that Uruguayan officials confiscated their banners and were unnecessarily slow.
Uruguayan authorities built a fence around the factory and brought in 500 extra police officers to guard the perimeter and to patrol the River Uruguay.
About 2,000 demonstrators finally gathered to sing the national anthems from both countries and express their anger at a factory they say will contaminate the rivers and drive away tourists.
The pulp mill, which officially opened last week, is the biggest foreign investment in Uruguay - a plant the Finnish company, Botnia, says employs the latest technology and will not pollute.
The dispute has gone to the International Court of Justice in The Hague and the Argentine and Uruguayan governments, despite a long and friendly history, are now barely on speaking terms.