Celebrities joined 800 protesters outside the Russian embassy in London to demand the release of 30 Greenpeace activists – including former Sheffield University student Kieron Bryan.
The 29-year-old video journalist has been charged with piracy after attempting to climb aboard an offshore oil rig in the Arctic Ocean, and faces a 15-year prison sentence if he is found guilty.
Blur lead singer Damon Albarn, film star Jude Law and former Clash bassist Paul Simonon all joined the demonstration.
Other protests were held in cities around the world calling for the release of the 30, including six Britons, who are being held in Murmansk.
Kieron studied in Sheffield for four years, taking a degree in English literature before completing a one-year course in broadcast journalism in 2008.
His family say he agreed to join the Greenpeace voyage to document it on film, not because he was a member of the organisation.
The activists were reportedly held at gunpoint when their ship the Arctic Sunrise was stopped by coastguards near a rig owned by the state-controlled Gazprom oil company.
Greenpeace says it was seeking to raise awareness of the potential environmental damage caused by drilling.