Greenpeace International campaigners scaled the anchor chains and ropes on the side of the ship and positioned small boats around the vessel to prevent it leaving the mill in northwest Finland.
"It's a crime that thousand-year-old ancient forests end up as glossy magazines on coffee tables across Europe," activist Matti Liimatainen said.
The M/V Antares was docked at the Veitsiluoto mill at Kemi, about 90 km (56 miles) from the Arctic Circle, loading paper for Finnish-Swedish Stora Enso, the world's largest paper and board maker.
"Stora Enso should stop buying wood from the Finnish state from areas which are of crucial importance for the reindeer- herding livelihood up in northern Finland and from areas which are ecologically valuable old-growth forests," Liimatainen told Reuters by telephone from the protest.
Green activists say the forests are an irreplaceable heritage and indigenous Sami reindeer-herders argue they are essential for the survival of their traditional way of life.
But Finland's state logging firm, Metsahallitus, which sells timber to local sawmills and paper firms like Stora Enso, maintains that existing protection for state-owned forests is sufficient.
The ship's owner, Finnlines, said it called the police after small craft blockaded the vessel, which was loading and scheduled to leave for Germany, Stora's single biggest market, later on Monday.
"We also received a written notice that there are buoys beneath the water surface ... to prevent the ship's navigation," said Antti Lagerros, chief executive of Finnlines.
Activists had been hanging from ropes and the anchor chain and one had got aboard but left when asked to by the ship's captain, Lagerros said.
Police were monitoring the situation.
Stora Enso said it believed both forestry and reindeer- herding should be maintained in northern Finland and was convinced that production from the region was sustainable.
"We feel ... that the ecological value of old-growth forests is already secured, because the protection rate is already so high," said Stora Enso's Susanna Rissanen.
The paper company would continue to buy wood from the state logging firm, she added.Planet Ark
