Greenpeace announced Friday that Executive Director of Greenpeace International Kumi Naidoo joined four other activists in anchoring themselves to the base of the rig in the arctic waters of the Pechora Sea.
The environmental advocacy group said two coast guard helicopters were circling the platform and personnel from the rig had doused them with ice-cold water from the rig deck.
The ice-resistant platform is the first to be designed and built in Russia. Russian energy company Gazprom said it paid "special attention" to environmental safety issues.
Critics of oil and natural gas exploration in arctic waters say they worry about the potential effects of a major oil spill.
Greenpeace, in a statement, said the Prirazlomnoye oil rig represents the "face of arctic destruction today."
Naidoo was arrested by Dutch authorities in June after scaling a platform belonging to Cairn Energy. He was barred from the Netherlands for one year.
A Dutch court ruled against Greenpeace after Cairn Energy sued the group.
Gazprom estimated reserves at the Prirazlomnoye oil field at around 527 million barrels, which the energy company said translates to roughly 43 million barrels worth of production per year.