GUALEGUAYCHU, Argentina (AFP) -- Tens of thousands of Argentines marched Sunday to a blocked international bridge to Uruguay, site of two paper mills they say jeopardize Argentina's environment and tourist industry.

The protest has halted traffic over the bridge since April 4, in an attempt to stall the 1.8 billion dollar Finnish-Spanish consortium project.

"The economic interests of managers of large finance corporations have undertaken projects that damage the environment and human lives," organizers said in a message delivered at the bridge over the Uruguay River, which defines the border between the South American countries.

"We denounce Uruguay for allowing the installation of the plants in violation of the Uruguay River Treaty," the statement said.

Organizers said that Argentine border police estimated that 100,000 protesters had gathered at San Martin International Bridge.

No protests were reported in Uruguay, which would reap the economic benefit from the plants and its government says that any environmental effects would be limited.

Protesters sang the national anthems of both countries as traffic backed up 32 kilometers (20 miles).

The environmental group Greenpeace also estimated crowd strength at 100,000 and in a statement urged both countries to enter talks to end the standoff.

Protesters drove in a caravan, led by firefighters on fire trucks, taking two hours to cover the 32 kilometers (20 miles) from the city of Gualeguaychu, 230 kilometers (143 miles) north of Buenos Aires.

Four hundred Argentine border police also braved the cold and wind, backed up by helicopter and border patrol launches in the river.

"Delegations came from six Argentine provinces and on Saturday the barricades were lifted to allow the Uruguayan contingent entry," Gualeguaychu tourism secretary Sebastian Bell told AFP.

In the Uruguayan town of Fray Bentos, 300 kilometers (185 miles) northwest of Montevideo, Botnia of Finland and Ence of Spain are building the plants, which protesters say will damage the environment and deter tourists. Environmentalists and residents blocked the bridge on April 4, over the objections of Argentine President Nestor Kirchner and of Uruguay's government, which called on Mercosur, a free-trade area comprising Argentina, Brazil, Paraguay and Uruguyay, to denounce the protesters methods.

Argentina has demanded a 90-day suspension of the paper mill project to conduct an assessment of how the mill could affect the environment.

"No to paper mills, yes to life," was the slogan as protesters walked through the town of 100,000.

Environmentalists and residents are to meet Monday to decide whether to take down the bridge barricade. Some are seeking promises from Argentina's government to take the case to the World Court in The Hague.