Jumping from helicopters and slithering down ropes, more than a dozen armed Russian coastguard workers boarded a Greenpeace ship and took custody of the activists on board, to stop them from disrupting the work of a controversial oil rig.

After a scuffle between the activists and the Russian security forces, the 29 activists, including six British nationals, are apparently being held on board at gunpoint, while the ship is forcibly towed to the Arctic port of Murmansk.

The Russian coastguard, which is controlled by the FSB security services, boarded the Arctic Sunrise late on Thursday night near Prirazlomnaya, a drilling platform in the Pechora Sea, close to the Novaya Zemlya archipelago.

The activists were protesting against the rig, operated by the Russian energy giant Gazprom, which is due to come online soon, and had attempted to climb aboard it and stop work.

The ship's crew remain in the custody of armed Russian security forces and could be charged with terrorism.

The FSB said it had been tracking the vessel since it left the Norwegian port of Kirkenes last Saturday, and turned off its radio signals. Once the ship had changed course and began heading for the Prirazlomnaya platform, the FSB decided to act. Warning shots were fired and two climbers on the rig were arrested earlier in the week.

When the ship's captain refused to turn back or respond to commands on Thursday, the FSB said it took the decision to act. About 15 armed men boarded the boat via helicopter, according to activists on board.

Ben Ayliffe, the head of Greenpeace International's Arctic oil campaign, said he was speaking to one of the activists via satellite phone during the storming, and could hear shouts and banging.

"They used violence against some of us. They were hitting people, kicking people down, pushing people," Faiza Oulahsen, one of the activists aboard the ship, said in a call to Reuters on Thursday evening.

Nothing has been heard from the activists since. The Russian coastguard said that the ship's captain was refusing to operate the ship, so an official boat was towing the Arctic Sunrise west towards Murmansk.

Greenpeace insists the ship was in international waters when it was boarded, and said there had been no formal notification of possible charges, nor offers of access to legal or consular assistance. The ship was 34 nautical miles from the closest Russian shore, according to the activists, which would put it in an area known as the Exclusive Economic Zone of Russia but not in the country's territorial waters.

The FSB said it was co-ordinating actions with the foreign ministry, Gazprom and oil company Rosneft "to protect the safety of the crew on the platform and defend the interests of the Russian Federation in the Arctic region".

The regional press office of the FSB in Murmansk told Russian agencies that it had received information from representatives of the Prirazlomnaya platform earlier in the week that they feared a terrorist act was about to be carried out, and said that activists were approaching the rig with an "unidentified object that looks like an explosive device". Greenpeace claimed this was disingenuous, as its "safety pod" is brightly coloured and branded with the organisation's logo.

Greenpeace has long warned that the start of oil drilling at Prirazlomnaya could have disastrous environmental repercussions. "The rig is a rusting hulk in the middle of the Arctic that is about to start pumping oil from the Arctic for the first time," said Ayliffe. "Gazprom has no way to clean up an oil spill if it happened, and it would cause huge damage to one of the most fragile natural environments on the planet."

The Arctic Sunrise ran a similar mission to Prirazlomnaya last year, and several activists again climbed on to the rig, but although they were observed by Russian authorities, there was none of the forceful reaction that occurred this time, Ayliffe said.

Vladimir Chuprov, the head of Greenpeace Russia's Arctic programmes, says the organisation is trying to arrange meetings with Russian officials to discuss the situation. A Greenpeace team is already in Murmansk awaiting the arrival of the boat, expected at some point on Monday.