Greenpeace claims two activists scaled the Prirazlomnaya oil platform today in protest over the operation in Arctic waters.
Production at the ice-resistant Gazprom-owned rig, Russia's first such project in the Barents Sea, was delayed last year after similar action. Gazprom put the delay down to "technical reasons".
Hydrocarbon production in its vast offshore areas is seen by Russia as vital to maintaining oil output, the world's largest, at no less than 10 million barrels per day this decade.
Global majors including ExxonMobil, Eni and Statoil have agreed deals with Russia's top oil producer Rosneft to enter the region, but their projects are not expected to begin extracting oil before the 2020s.
Greenpeace said in an emailed statement it sent five boats to the rig early today. One was arrested by the Russian coastguard, which fired warning shots across the bows of its Artic Sunrise base vessel. It said two of its members climbed the rig and would remain there as long as possible.
"Despite massive financing for Prirazlomnaya, it is not able to guarantee safe production of Arctic oil," Greenpeace said.
Offshore safety concerns have grown after BP's Deepwater Horizon oil rig exploded in 2010, killing 11 workers and spewing millions barrels of oil into the Mexican Gulf.
Prirazlomnoye is the first Arctic offshore oil deposit to be developed by Russia and is located in the Pechora Sea, a part of the Barents Sea, 40 miles from the northern coast. It is expected to reach peak production of 6 million tonnes per year (120,000 barrels per day) in 2019.
Gazprom Neft, the oil arm of state gas export monopoly Gazprom, is expected to obtain a licence to develop the field. It expects overall investment in the project to be around 200bn roubles (£3.9bn), of which half had already been spent. Gazprom Neft and Gazprom declined immediate comments.