UPDATE 3-Russia to prosecute Greenpeace Arctic oil rig activists
* Activists could face piracy charges, investigators say
* Diplomats visit detained protesters on ship
* Greenpeace accuses Russia of illegality (Updates with diplomatic visit)
By Thomas Grove and Liza Dobkina
MOSCOW/ST PETERSBURG, Sept 24 (Reuters) - Environmental activists who protested at an offshore oil platform in the Russian Arctic last week will be prosecuted, possibly for piracy which is punishable by up to 15 years' jail, Russian investigators said on Tuesday.
They said the "attack", in which Greenpeace activists tried scaling the Gazprom-owned Prirazlomnaya platform, Russia's first offshore Arctic oil platform, had violated Russian sovereignty.
"When a foreign ship full of electronic equipment intended for unknown purposes and a group of people, declaring themselves to be environmental activists, try to storm a drilling platform there are legitimate doubts about their intentions," the investigators said in a statement.
The protest ended when armed officers boarded the Netherlands-registered icebreaker and arrested the 30 activists onboard, Greenpeace said. The vessel, the Arctic Sunrise, was towed to the port of Murmansk.
The activists and crew were escorted off the ship late on Tuesday and taken to a local Investigative Committee office, Greenpeace spokeswoman Marina Favorskaya said.
Greenpeace says its protest - aimed to draw attention to the threat oil drilling poses to the fragile Arctic eco-system - was peaceful, and that Russia's actions violated international law.
A diplomatic delegation of 18 officials representing nine countries went on board the Arctic Sunrise on Tuesday afternoon to meet the detainees, Greenpeace said.
The Netherlands, where Greenpeace International is based, called for the release of the ship and all activists, including two Dutch citizens.
"The sailors and the ship have to be released," Defence Minister Jeanine Hennis-Plasschaert told parliament.
Greenpeace said the activists had been denied access to lawyers and that they had not yet been formally charged.
Greenpeace's global chief Kumi Naidoo said: "Any charge of piracy against peaceful activists has no merit ... we will not be intimidated or silenced.
"Peaceful activism is crucial when governments around the world have failed to respond to dire scientific warnings."
Greenpeace says scientific evidence shows that an oil spill from Prirazlomnaya would affect more than 3,000 miles of Russia's coastline.
Onshore drilling is well established, but significant offshore work is in its infancy despite relatively shallow waters and numerous attempts.
A decade of high oil prices, scarcity of opportunities elsewhere and a shrinking ice cap have led companies to look to unexploited parts of the Arctic in recent years.
Global majors including ExxonMobil, Eni and Statoil have agreed deals with Russia's state-owned Rosneft to enter Russia's Arctic offshore waters. (Editing by Alissa de Carbonnel and Robin Pomeroy)