"We're not going to permit any outsiders from coming and solving the problems of Uruguayans," Vazquez said in an interview with local radio.

"We Uruguayans have never let ourselves be 'roughed up', to use a popular expression, and this government and its people will not be roughed up," he said.

The arrests were the first made in months of protests by Argentines living next door to the future mills, which with a combined investment of $1.7 billion, are Uruguay's most costly industrial project to date.

Police released all nine activists after a few hours.

Residents say the plants will pollute the air and hurt the wildlife along the Uruguay River, shared by the two countries.

Greenpeace said protesters from several countries - including Finland, Italy, Germany and Mexico - took a boat across the river that separates Argentina and Uruguay and occupied a dock belonging to Botnia.

The organization vowed to keep up its protest actions.

Metsa-Botnia, Europe's second-largest pulp producer, plans to begin exporting 1.5 million tonnes of wood pulp for export as of 2007 and Ence is scheduled to start operating in 2008.

Argentines opposed to the Europeans' plans have staged massive road blockades for the past few weeks, preventing tourists from crossing the border into Uruguay during the high tourist season.

The companies have argued they are using the most advanced technology available to minimize the impact on surrounding areas and have the support of the local government.

"We don't understand the problem. We have all the permits and have complied with the highest environmental standards ... The highway blockades don't affect us directly, they just raise the tone of the conflict," said an Ence official in Madrid.