Greenpeace activists have launched an attempt to board a Russian oil platform and prevent it drilling in the Arctic.
The group released pictures of two activists scaling the side of the giant Prirazlomnaya platform, and of security forces boarding one of its boats.
The project, run by oil monopoly Gazprom, is Russia's first effort to extract oil from the Barents Sea.
Greenpeace opposes Arctic drilling, saying it threatens a unique and fragile environment.
The group said two of its activists were arrested and were being held on a Russian coastguard ship.
It said the coastguard vessel had fired 11 shots across the main Greenpeace ship.
Russian authorities did not comment on the Greenpeace reports.
Prirazlomnaya is scheduled to begin production by the end of the year. Russia's economy and recent growth depends on income from its huge oil and gas deposits.
A Greenpeace statement said: "Despite massive financing for Prirazlomnaya, it is not able to guarantee safe production of Arctic oil."