By Daniel Schweimler
BBC News, Buenos Aires
Tens of thousands gathered in the Argentine town of Gualeguaychu for what organisers called the country's biggest-ever environmentalist rally.
The dispute between the two countries has lasted over a year
They are trying to halt the construction of two paper pulp mills on the Uruguayan side of the river that separates the two countries.
But most Uruguayans want the work to continue and the conflict is driving two normally friendly nations apart.
The dispute has been raging for over a year and shows no sign of going away.
They came from all over Argentina to pledge their support.
Tens of thousands of people carrying Argentine and Uruguayan flags and banners reading "No to the pulp mills" walked to the 5 km-long bridge that links the two countries over the River Uruguay.
It is on the Uruguayan side of that river that two European companies have started building two pulp mills.
The protesters, backed by international environmentalist groups such as Greenpeace, say the mills will pollute the river.
They have been blockading the bridge for several months and the Argentine government is taking the case to the International Court of Justice in the Hague, accusing Uruguay of breaking a treaty governing the protection of the waters.
The builders say they are employing the latest technology and the Uruguayan people want the jobs and investment the projects, backed by the World Bank, are bringing.
Relations between the governments in Buenos Aires and Montevideo have reached rock bottom, causing concern in neighbouring countries.
But there appears to be no room for compromise and the bridge that was supposed to bring Argentina and Uruguay closer together is fast becoming a symbol of what divides them.