Actor Jude Law and musician Damon Albarn condemned charges of piracy against 30 Greenpeace activists following an Arctic oil rig protest as "ludicrous" as they joined worried relatives in a vigil outside the Russian Embassy.
Both men, along with Paul Simonon of the band The Clash, said the arrests were an international disgrace and a personal blow as they are friends with detainee Frank Hewetson.
Law said he was "exercising my right to peacefully protest" as he joined around 800 people at the central London vigil.
It is part of several being held across Britain and in 47 countries in support of the 30 people, including six Britons, who are now in Russian custody. The aim is to keep the pressure on the Russian authorities.
The Russian coast guard seized the ship Arctic Sunrise and everyone on board following the September 18 protest at the offshore platform owned by Russia's state-controlled energy giant Gazprom in the Pechora Sea.
The activists are now in custody in the northern city of Murmansk.
Law, whose children went to school with Mr Hewetson's, said: "I am just adding my face and body to the mass of support.
"The fact that there is a threat of conviction did not put them off.
"What is ludicrous is that they have been charged with piracy which has a threat of 15 years in prison.
"Of course I am worried about Frank because I care about his family and I care about him but I know that he is incredibly durable.
"I think that it is very interesting that the people over there probably knew there would be an arrest involved and the threat of a conviction is probably part and parcel of the act of drawing attention to the drilling in the Arctic which we all know is an international problem which needs confronting."
Protesters stood quietly behind banners declaring "Free the Climate Defenders", "Journalist and Not Pirate" and "Free The Arctic 30".
British freelance videographer Kieron Bryan and UK activists Mr Hewetson, Philip Ball, Alexandra Harris, Anthony Perrett and Iain Rogers have been charged over the protest.
To charge them with piracy is "ridiculous", Albarn said.
"It does seem to be a slightly different idea in Russia's collective head about what activism actually represents.
"Nine times out of 10 people who protest peacefully, whether it is through music or through trying to scale oil rigs, are doing it because they believe they are saying something that will ultimately benefit society as a whole.
"You have to remember that when the Russian government talks with this kind of rhetoric, it is actually being anti-humanitarian. And that is a shame because we put people in power to look after us presumably, but unfortunately rarely do they."
He went on: "I want to express my support for Frank, his family and all the other families because at the end of the day this is a very human thing.
"There are people who are terribly worried about their loved ones.
"Everyone is concerned because it is Russia and they have a very bad record of bowing to international pressure.
"They need to understand that these people are not a threat. This is something that is very human. It is not a political issue."
Simonon said: "This is a protest to show support. There is no reason why it could have been called piracy. "I am here to show my support to Greenpeace and the families."
Actors Imelda Staunton and Jim Carter later joined the protest.
The families of Mr Bryan and Ms Harris travelled from their homes in Devon for the vigil.
Mr Bryan's mother, Ann, said: "I am sick with worry; at least a protest like this makes me feel like we are not alone.
"None of the 30 should be facing that charge. Kieron was just doing his job."
Mr Bryan's father, Andy, said: "We do not want this attention to go away. We are coping with this day by day."
Ms Harris's sister, Georgina, said she had received an email from her sibling saying she was safe and warm and urging her not to worry.
Ms Harris said her sister seemed "emotional" and added: "It is hard not to worry considering everything that is going on.
"I am trying to stay positive but this is disgusting."
Greenpeace denies any wrongdoing and described the charge as "absurd and outrageous".
Campaigners wrote messages of "peace and solidarity" on paper doves, which are to be sent to those detained in Murmansk.
Carter described it as "just a ludicrous situation", adding that Prime Minister David Cameron, plus the leaders of all the 18 other countries who make up the nationalities of the 30 activists, should apply pressure to Russia.
The actor said: "I can't believe the Russians are behaving like this whereby people exercising their right to peaceful protest, armed only with good principles and a banner, are accused of piracy and facing possibly up to 15 years in jail - it is just ridiculous.
"Any right-thinking person from any side of the political spectrum has got to agree with this - there cant't possibly be any disagreement.
"I hope all of the politicians from David Cameron down to everyone who represents these 30 people should get on to President Putin immediately and protest about it."
Staunton described herself as being "as outraged as everyone else that the Russian establishment has decided to call these peaceful protesters pirates".
She said: "It is a global issue. I am very worried but also hopeful that the pressure that hopefully we can put on Russia will have some effect.
"The pressure is trying to get them to see sense that Greenpeace is a globally-recognised peaceful organisation. They are not pirates.
"Demonstrations are being held all over the world and all these things help."