Masked men have broken into Greenpeace’s Russian headquarters in an apparent attempt to intimidate the environmental group into halting its demonstrations against the detention of 30 people over protests against Arctic drilling.

Twenty-eight Greenpeace activists and two freelance journalists were detained last month over the protest at an Arctic oil rig.

They group, dubbed the Arctic 30, face piracy charges which are punishable by up to 15 years’ jail.

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Security camera footage posted on the group’s website today showed six balaclava-wearing men climbing a fence and running into a guarded car park at Greenpeace’s Murmansk headquarters overnight on Thursday. They stole a mock cage that Greenpeace protesters had planned to stand in outside the Murmansk Regional Court yesterday in protest against the detentions. According to Greenpeace, the burglars also slashed the tyres of a car.

“They probably had noticed our activists’ car earlier, and they slashed the tyres of another car that looks a lot like it, which by accident was parked in the space where the activists usually leave theirs,” Greenpeace said in a statement. Two women who witnessed the theft were threatened by the burglars, Greenpeace added.

The news agency Regnum said local police were investigating the theft, which came ahead of a day of protests against the detentions across Russia today.

Meanwhile, the Murmansk Regional Court denied bail to a British activist, Alexandra Harris, as well as three other crew members and activists from Canada, Denmark and the Netherlands. Among the six Britons detained, Philipp Ball, Kieron Bryan, Frank Hewetson, Anthony Perrett and Ms Harris have now been denied bail. MPs are planning to debate the detentions on 23 October after their families appealed to the Foreign Office.

Thousands protested against the activists’ detention yesterday at more than 100 events in 36 countries, according to Greenpeace. At least 100 people, many of them activists and journalists, gathered in a fenced-off “Hyde Park” controlled demonstration space in Gorky Park in Moscow.

After speaking at the protest, Yevgenia Chirikova, Russia’s best-known environmental activist, criticised world leaders who had “shut up like a clam” instead of defending their detained citizens.

“I expect we’ll have enough strength of will to hold this action, and we will be braver than the prime minister of the United Kingdom, for instance, who hasn’t said anything in defence of his six citizens now in confinement for defending the Arctic,” Ms Chirikova told The Independent. The German Chancellor, Angela Merkel, is the only head of government to have raised the activists’ detention with President Vladimir Putin, urging a quick resolution to the case.

Olga Kuptsova, who was one of 30 people at the protest holding signs with common “lies” about the Arctic Sunrise case, most of them dressed in orange prison jumpsuits, said she came out to try to show Russians that Greenpeace was not the “extremist organisation” many think it is.

She told The Independent: “Lots of people, even people I know who seem sensible, when you start talking to them about this situation they for some reason don’t side with these activists.”