Demonstrations have spread from the border to the Argentine capital

The protests at the three international bridges on the Uruguay river were aimed at "forcing an end to construction", said ambassador Hector Gros Espiell.

Mr Espiell made the remarks before the International Court of Justice (ICJ) which is deliberating on the case.

Argentina maintains the mill will pollute the Uruguay river.

In July, the ICJ threw out Argentina's request to have the work stopped pending a final court ruling.

And last month, the World Bank approved more than $500m (�259m) in loans for the project, after a study found the plant met its environmental standards.

Mr Espiell, Uruguay's ambassador to France, told the UN tribunal at The Hague that Uruguay was confronted by an urgent crisis.

"There is no legitimate excuse which can justify the omission of the Argentine government in its duty to prevent the blockade," the ambassador told the ICJ hearing.

The wave of Argentine protests against the project has already caused Ence, the Spanish company that was developing a second pulp mill for the site, to relocate its project elsewhere in Uruguay.

But the mill being built by Finnish company Botnia is going ahead.