A group of 13 protesters from Greenpeace International stormed UPM-Kymmene's headquarters to protest against the group's involvement in a nuclear reactor.
UPM-Kymmene is the largest private shareholder in Teollisuuden Voima (TVO), which has applied for permission to build a fifth nuclear reactor in Finland, on the Baltic coast.
Greenpeace spokesman Mike Townsley said: "We felt we had to make the point that this project was leading Finland down an unsustainable energy path.
"Three activists abseiled from the roof, while eight built a brick wall in front of the office," he said.
The protestors unveiled a 20m-long banner from the roof and the brickbuilders barricaded themselves in for four hours before all were arrested.
"UPM-Kymmene should now have the vision of looking to the future instead of clinging to the past," said campaigner Harri Lammi.
A UPM spokesman admitted a protest had taken place, and that the banner remained in place for several hours before it was removed.
Greenpeace said UPM-Kymmene, as the largest shareholder, would decide whether or not an application for the reactor would be handed in.
Story by Andy Scott
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