Greenpeace activist Evangelina Carrozo, a carnival queen from the Argentine town of Gualeguaychu, headed the rally outside the Finnish Embassy in downtown Buenos Aires. Some hoisted signs that read "No to the paper mills!" The project is being built by Finland's Metsa-Botnia and Spain's Ence.
On Friday Carrozo interrupted a group photo at the EU-Latin America meeting in Vienna dressed in a tasseled bikini and thigh-high leather boots to protest the two mills.
Worried about pollution, Argentina has called for construction to be halted on the mills, part of US$1.7 billion pulp project along the Uruguay river that borders both countries.
Uruguay defends the country's biggest industrial investment, saying the companies will use the latest technology to keep pollution within internationally accepted levels.