The actor Jude Law and the musician Damon Albarn were among hundreds of supporters to join family members protesting yesterday against Russia's continuing detention of Greenpeace activists, outside the Russian embassy in west London.

Law, whose children went to school with those of Frank Hewetson, one of six Britons currently being held in Russia, said that the charges against him and the other detainees were "ludicrous". In all, 30 activists from various countries were arrested after protesting against drilling in the Arctic.

"Frank is a friend, so I am worried about him and his family," the actor said. "Drilling in the Arctic is an international problem that needs to be confronted, and those held would want us to remember that."

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Albarn, who plays with Blur and Gorillaz, called the charges "ridiculous" and said that he had his "fingers crossed" that the protest by around 800 supporters would have an effect. But he added there would "probably still be a lot of back-breaking bureaucracy to get through" before any of the detainees were released.

Albarn said that he had met Mr Hewetson a number of times and that Russia needed to release him and the other detainees. "I want to express my support for Frank, his family and all the other families," he said. "These people are not a threat. It is not a political issue."

The families locked arms with friends of the six detained British nationals – the videographer Kieron Bryan and UK activists Mr Hewetson, Philip Ball, Alexandra Harris, Anthony Perrett and Iain Rogers – and stood defiant outside the embassy.

The Greenpeace activists were arrested after Russian coast guard seized the ship Arctic Sunrise and all on board following the 18 September protest, in what Greenpeace international director Kumi Naidoo yesterday called the worst assault on the group's environmental activism since its flagship Rainbow Warrior was bombed in 1985.

Outside the embassy, some wore T-shirts with messages of support, others held placards showing the faces of the detained. They were joined by members of the public holding banners calling on Russia to "Free the climate defenders" and "Bring home the Arctic 30".

Alexandra Harris's sister Georgina, 23, said that the protest, one of dozens of such vigils worldwide yesterday, had made her feel "much better". She said: "It opens your eyes to the fact that there are people out there supporting us."

Ms Harris said that she had received an email from her sister, telling her family to "be strong" and that she was "OK, but emotional".

The family and demonstrators wrote messages of love and support on paper doves to be sent to Murmansk, Oblast, where the detainees are being held. Cars and buses full of commuters honked and cheered as they passed the protest.

Ann Bryan, mother of Kieron Bryan, called the support for families "overwhelming", and added that while she felt "sick with worry" the action "gave them hope".

Steve Ball, the brother of Philip Ball, who attended with his mother, Linda, said that the backing they had received from around the globe, including from designer Dame Vivienne Westwood, Downton Abbey actor Jim Carter and actress Imelda Staunton, who were all outside the embassy yesterday, had been "fantastic".

"The charges are not right, but we are pretty confident that at some point they [the Russians] will have to back down," he added.