* Greenpeace says guns pointed at activists' heads
* Russia calls oil platform protest "aggressive"
* Summons Dutch ambassador to complain (Releads with boarding)
By Gabriela Baczynska
MOSCOW, Sept 19 (Reuters) - Armed Russian coastguard officers boarded a Greenpeace ship that was circling an Arctic oil platform on Thursday, after Moscow accused the environmentalist group of "aggressive and provocative" behaviour.
Greenpeace said coastguard personnel rappelled onto the ship from a helicopter. "Greenpeace International activists locked inside the radio room said they saw other activists detained on their knees with guns pointed at them," it said in a statement.
A tweet that Greenpeace said was posted from on board the ship before communications were cut, read: "This is pretty terrifying. Loud banging. Screaming in Russian. They're still trying to kick in the door."
Two Greenpeace activists scaled the side of the Gazprom-owned Prirazlomnaya platform on Wednesday and were arrested for actions the Russian Foreign Ministry said threatened security.
Moscow summoned the Dutch ambassador to discuss the issue. Greenpeace is based in the Netherlands and its Arctic Sunrise, which had 25 people aboard, is a Dutch registered vessel. The two people arrested for boarding the rig are of Swiss and Finnish nationality.
The protest is against the environmental risks posed by increased energy exploitation in the Arctic, estimated to hold 13 percent of the world's undiscovered oil reserves and 30 percent of its undiscovered natural gas.
Prirazlomnaya is Russia's first Arctic offshore oil rig and a sensitive project in a country that has made tapping the region's resources a priority. Greenpeace activists last boarded the platform in August.
"The intruders' actions were of aggressive and provocative character and had the outward signs of extremist activity that can lead to people's death and other grave consequences," the Russian foreign ministry said in a statement issued before news of the armed raid on the boat.
Greenpeace International executive director Kumi Naidoo said: "We ask President Putin to restrain the Coast Guard and order them to holster their guns and withdraw. We are a peaceful organisation and our protest has done nothing to warrant this level of aggression."
Global majors including ExxonMobil, Eni and Statoil plan to work in Russia's Arctic waters and Norway is pushing ahead with its own drilling plans.
But technical difficulties, high costs and mishaps as well as environmental campaigns have caused hesitation among some players.
Developing the Prirazlomnoye oil deposit, in the Pechora Sea some 60 km (40 miles) from Russia's northern coast, was delayed last year, with Gazprom blaming technical problems. It is expected to reach peak production of 6 million tonnes per year (120,000 barrels per day) in 2019. (Editing by Ralph Boulton)