Protests were held in cities across Britain and around the world on Saturday to show support for the 28 Greenpeace activists and two journalists currently being held by the Russian authorities on charges of piracy. Even as the vigils were taking place, the Russian authorities shrugged off the controversy, saying that the group's protest at an Arctic oil platform owned by state-controlled firm Gazprom had been "pure provocation".

Six Britons are among the group, drawn from 18 countries, being detained in jail in the northern Russian city of Murmansk after being seized at gunpoint last month along with their ship, the icebreaker Arctic Sunrise. Greenpeace says the activists had been protesting peacefully in international waters to highlight the environmental cost of drilling in Arctic waters.

The global day of solidarity included a gathering of an estimated 800 people at the Russian embassy in London. Among the protesters were actor Jude Law and musicians Damon Albarn and Paul Simonon, who are all friends of Frank Hewetson, one of those being held; and the fashion designer Vivienne Westwood.

Other protests took place in Spain, France, New Zealand, Russia and in Hong Kong, where a human banner was formed reading: "Free the Arctic 30." The government of the Netherlands announced on Friday that it would launch legal action to free the 30, two of whom are Dutch, and the vessel which is Dutch-flagged.

The Dutch foreign minister, Frans Timmermans, said the Netherlands had applied to the UN's Tribunal for the Law of the Sea, and that his country, the first nation to take legal action in the case, viewed the ship's detention as unlawful. "I don't understand why this could be thought to have anything to do with piracy, I don't see how you could think of any legal grounds for that," said Timmermans.

Under the rules of the tribunal, the Netherlands can ask for the immediate release of the ship and its passengers, which include a British film-maker, Keiron Bryan, and a Russian photographer.

The ship's captain is Peter Willcox – the veteran American environmentalist who commanded the Rainbow Warrior when it was sunk by French secret service agents in New Zealand in 1985.

But the Russian deputy foreign minister, Alexei Meshkov, said Russia had repeatedly asked the Netherlands to halt "illegal activity" by the ship. "Unfortunately, this was not done. Therefore, we have far more questions for the Dutch side than they can have for us. Everything that happened with the Arctic Sunrise was pure provocation." Piracy carries a prison sentence of up to 15 years.

Greenpeace International's executive director, Kumi Naidoo, said: "The activists were taking a brave stand to protect all of us from climate change and the dangers of reckless oil drilling in the Arctic. Now it's imperative that millions of us stand up with them to defend the Arctic and demand their immediate release."