Uruguayan immigration and customs officials inspected each protester's car, letting them cross after seizing dozens of sticks used as poles to carry Argentine flags.

They said about 800 demonstrators in 220 vehicles took part in the demonstration.

"No to the paper plants!" read signs flown from car windows as the protest caravan arrived in the city of Fray Bentos on the Uruguay side of the border.

'Deeply concerned'

Omar Lafluf, a Fray Bentos government official, said residents are "deeply concerned" about the Argentine protesters entering Uruguay.

"It's clear after clear after such a long process of auditing that the mill has no environmental impact but has a very big economic effect"

Erkki Varis,
Botnia timber company chief executive
Uruguay has denied that the plant, which is due to open later this month, will pollute Argentina and says it is key to the small South American nation's economic development.

Erkki Varis, chief executive of the Botnia timber company, said: "It's clear after such a long process of auditing that the mill has no environmental impact but has a very big economic effect."

The plant is being built by a Finnish consortium. At $1.2bn it is the largest foreign investment in the country and could create up to 8,000 jobs.

But Argentina's government sided with the environmentalists and took the case against the facility to the International Court of Justice.

They also argue that the plant violates a river protection treaty.