Actors Imelda Staunton, 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.
The stars 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.
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The US president is seeking an investigation by the Justice Department.
The casualty was taken into the care of paramedics with a suspected leg injury.
The 16-year-old was discovered with stab wounds on Thursday afternoon.