Argentine President Nestor Kirchner has rallied crowds at a demonstration against two paper pulp mills under construction in neighbouring Uruguay.
Protests over the mills have continued for several months
He told the rally in the border town of Gualeguaychu that Argentines were "standing together" to solve the issue.
His words came a day after Argentina filed a formal complaint over the mills at the International Court of Justice in The Hague.
Argentina fears the mills will cause extensive pollution and deter tourism.
But Uruguay says the mills are safe and will provide much needed jobs. The row between the two countries has been going on for several months.
The mills are being built in the city of Fray Bentos by Finland's Metsa-Botnia and Spain's Ence. Together they represent the largest-ever industrial investment in Uruguay.
Mr Kirchner addressed a crowd of thousands in Gualeguaychu, a town on the river close to the site in Uruguay where the mills are being built which has been the site of months of protests.
"This is an environmental issue that concerns both Argentines and Uruguayans," he said.
He criticised the Uruguayan government. "They say the impact will be minimal but they won't give us sufficient information or carry out studies to clear up the question," Mr Kirchner said.
In the lawsuit filed in The Hague, Argentina called for the project to be suspended, saying that adequate studies on the environmental impact had yet to be carried out.
Last month, the World Bank delayed a decision on about $400m (£229m) of funding for the mills, saying it needed more information on the potential environmental impact of the facilities.
On Sunday, thousands of demonstrators gathered in the town for what organisers called the country's biggest-ever environmentalist rally.