Sculptor Anish Kapoor has condemned China's "barbaric" detention of artist Ai Weiwei and has called on galleries to close for a day in protest. Ai - an outspoken critic of China's human rights record - has not been heard from since he was arrested in Beijing at the start of April. Kapoor has dedicated his monumental Leviathan art installation in Paris, unveiled on Tuesday, to Ai. Artists must speak with "a communal voice", he told BBC Radio 4's Today.