Dutch police stormed a Greenpeace ship on Thursday to prevent environmental activists blocking delivery of the first oil from Russia's new Arctic drilling platform reaching port in Rotterdam.
The Rainbow Warrior was crewed by the activists who were detained last year by Russia in the Arctic, the campaign group said. Greenpeace is opposed to drilling in the Arctic Sea which it says risks causing a catastrophe in a fragile ecosystem.
Police said the activists had reneged on an agreement they had made with harbour authorities not to interfere physically with the ship during their protest.
A Reuters photographer said activists had draped banners saying "No Arctic Oil" from the Russian vessel.
"The Russian ship is very big, about 250 metres long, and there are safety concerns when you try and stop it mooring," Rotterdam police spokesman Roland Ekkers said.
He said the activists had been detained in a room on the Rainbow Warrior until it docked, when the captain was arrested. The oil-tanker Mikhail Ulyanov entered the harbour unhindered, and moored at about 0915 GMT.
"Arctic oil represents a dangerous new form of dependence on Russia's state-owned energy giants at the very moment when we should be breaking free of their influence," said Greenpeace executive director Kumi Naidoo in a statement.
Tensions between Russia and Ukraine have prompted many analysts to warn that Europe is over-dependent on Russian gas, with some saying that the continent's reliance on Russia for energy makes it too costly to impose sanctions on the country.
"Thirty of us went to prison for shining a light on this dangerous Arctic oil," Dutch activist Faiza Oulahsen said in the Greenpeace statement.
The vessel, which had come from the Russian Arctic port of Murmansk, according to Thomson Reuters data, is carrying some 70,000 tonnes of oil from Gazprom's Prirazlomanaya oil platform in the Arctic Pechora Sea. The platform was briefly occupied by Greenpeace activists last year.
They were arrested by Russian military forces and charged with piracy, carrying a potential prison term of decades, but released under an amnesty initiated by President Vladimir Putin.
(Reporting by Michael Kooren; Writing by Thomas Escritt; Editing by Louise Ireland)