CCTV footage released by Greenpeace shows a number of men in balaclavas reportedly break into their offices in Murmansk, Russia.
The group said a mock cage, due to be used in a stunt as part of a campaign today, was taken from their premises.
Russian authorities are holding 30 activists and journalists following the capture of their boat during a stunt to highlight the environmental impact of Arctic drilling.
A court in Russia has charged the activists, and accompanying journalists, with piracy. They are currently imprisoned in jails in Murmansk, in the very north of Russia, and could face up to 15 years in jail.
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Temperatures are predicted to reach a high of 23C.
Hunt accused companies of "turning a blind eye" to a generation of children "exposed to harmful emotional side effects" of social media.
Top temperature 24C/75F.