The great turbine hall at London’s Tate Modern gallery, a former power station, is a notoriously difficult space for an artist to fill with authority. Its immensity can dwarf the imaginations of all but a select tribe of modern artists who understand the mysteries of scale, of how to say something interesting when you also have to say something really big. Louise Bourgeois’s giant spider once stood menacingly in this hall; Anish Kapoor’s Marsyas, a huge, hollow trumpet-like shape made of a stretched substance that hinted at flayed skin, triumphed over it majestically.

Last October the leading Chinese artist Ai Weiwei covered the floor with his Sunflower Seeds installation: one hundred million tiny porcelain objects, each hand-made by a master craftsman, no two identical. Sunflower Seeds is a carpet of life, multitudinous, inexplicable, and in the best Surrealist sense, strange. The seeds were intended to be walked on, but further strangeness followed. It was discovered that when trampled they gave off a fine dust that could damage the lungs. These symbolic representations of life could, it appeared, be dangerous to the living. The exhibit was cordoned off and visitors had to walk carefully around the perimeter.

Art can be dangerous. Very often artistic fame has proved dangerous to artists. Ai Weiwei’s work is not polemical – like the Sunflower Seeds, it tends towards the mysterious – but his immense public prominence (he was a co-designer of the Bird’s Nest Stadium for the Beijing Olympics, and was recently ranked at 13 in Art Review magazine’s list of the 100 most powerful figures in art) has allowed him to take up human rights cases and to draw attention to China’s often inadequate responses to disasters (the plight of the child victims of the Sichuan earthquake or those afflicted by the huge fire at Jiaozhou Road, Shanghai). He has embarrassed the authorities and been harassed by them before, but now they have gone on the offensive against him.

On April 4, Ai Weiwei was arrested by the Chinese authorities as he tried to board a plane to Hong Kong and disappeared. His studio was raided, computers and other items were removed. Since then, the regime has allowed hints of his “crimes” to be published – tax evasion, pornography. These accusations are not credible to those who know him. It seems that the regime, irritated by the outspokenness of its most celebrated art export, whose renown has protected him up to now, has decided to silence him in the most brutal fashion. On the same day, Wen Tao, a freelance journalist and one of Ai’s partners, was kidnapped by several unidentified persons on a street in Beijing, but the police have refused to say who is responsible for his disappearance.

The disappearance of Ai Weiwei is made worse by reports that he has started to “confess”. His release is a matter of extreme urgency and the governments of the free world have a clear duty in this matter.