Two Gazprom helicopters hovered over the group and periodically sprayed them with pressurised streams of ice water as they hung down on ropes from the side of Gazprom's huge red Prirazlomnaya platform, due to start commercial operations next year.
"They're hosing us," Greenpeace International's Executive Director Kumi Naidoo tweeted, while the team held up bright yellow signs reading "Save the Arctic!" and "Stop Gazprom!"
Naidoo said he did not expect coast guards to reach the remote spot of the inhospitable sea until Saturday and posted pictures of the team setting up swinging tents in which they planned to spend the night and have dinner.
The daring raid comes as Russia takes the lead from other Arctic energy powers in exploiting previously untouched territory for what is believed to be one of the world's largest holdings of recoverable oil and natural gas.
Gazprom's independent project is due to kick off next year just as fellow state oil firm Rosneft begins its own initial explorations with new partner ExxonMobil.
The area -- also the subject of territorial rows with resource rivals Canada and Norway -- is becoming especially attractive as the size of the ice shelf shrinks and conflicts continue to rattle energy producers in the Middle East.
Greenpeace said its team reached Gazprom's floating production base by launching a pre-dawn sneak attack by inflatable speedboats from its ship Arctic Sunrise and then climbing aboard using mooring lines.
Footage shot by one of the crew showed the sea calm but draped in metallic clouds as the tiny bright orange craft sped through unguarded waters toward the towering crane-mounted station.
"Six climbers have taken up positions on the structure and have interrupted the platform's operations," the group said in a statement.
The state-owned firm immediately denied any impact on the platform's operations and said the activists had turned down an offer to enter the base for talks.
"They were invited aboard the platform for a constructive dialogue," a Gazprom spokesman told Russian news agencies.
"But they refused and said they would prefer to hang off the platform instead."
Gazprom next year will become the first company to start commercial drilling in the Arctic when it launches offshore operations in the southeastern section of the Barents Sea.
The holding's base runs just west of the developments being pursued jointly by ExxonMobil and Rosneft in an area viewed by the Kremlin as the main source of Russia's oil and gas in the new century.
But critics warn that Gazprom's drilling is extremely risky because the platform is sealed in ice for most of the year and has to work smoothly in temperatures that often plunge to minus 50 degrees Celsius (minus 58 Fahrenheit).
The Gazprom unit plans to drill and process oil before injecting it into tankers -- operations that have never been performed in such an inhospitable climate before.
Critics say the risk of such work far outweighs the benefits it may offer either the Russian government or consumers through cheaper fuel.
"The Prirazlomnaya platform will produce no more than seven million tonnes of oil a year," Greenpeace Russia director Vladimir Chuprov told Moscow Echo radio.
"And the country needs to produce 500 million tonnes a year."