Tens of thousands of Argentines have marched to the Uruguayan border in one of the biggest protests so far against a controversial pulp paper mill.
Protesters say the mill will pollute the river
They have been demonstrating against the construction of the factory for more than two years, saying it will pollute the environment.
This week Uruguay President Tabare Vazquez said the mill could start work.
Correspondents say this has plunged Mr Vazquez' already difficult relationship with Argentina to a new low.
Protesters marched waving banners that read "no to the paper plant".
Thousands of cars hooted as they crawled towards the bridge that links Argentina with Uruguay and from where they could see the huge Finnish-built factory they say will pollute the river that separates the two countries.
Protest boats took to the water and demonstrators fired flares and shouted through megaphones.
"We have lost the battle, but not the war," protester Javier Castel told the Associated Press news agency.
They have been demonstrating for more than two years in a bid to stop the plant being built.
Argentina has taken the case to the International Court of Justice in the Hague and is awaiting a final ruling.
But Uruguay says the $1.2bn enterprise - the biggest in the country's history - is using the latest technology and will not pollute.
Uruguayan police are guarding the plant and have partially closed the border.
The dispute cast a shadow over the Ibero-American summit in Chile, where Mr Vazquez gave approval for the pulp mill to start operating.
His Argentine counterpart, Nestor Kirchner, accused him of punching the Argentine people in the back.
The BBC's Daniel Schweimler in Buenos Aires says the two men had hugged earlier in a rare show of friendship, but now they are again snarling at one another.