Ai Weiwei, a prominent Chinese artist and architect, has disappeared since being detained by the authorities at the Beijing Airport on April 3. Ai, whose “Sunflower Seeds” installation is on display at the Tate Modern gallery in London, has been called China’s “most famous artist.”

Ai has long been on the radar of the Chinese state department for the criticisms of China’s communist rule often found in his work, though he has been protected from violent action by both his prominent international presence and his father’s status as a respected Chinese poet. Beijing authorities searched Ai’s studio and home on Sunday night, temporarily detaining his wife and several of his employees. At least one of the employees remains detained.

The United States, Germany, France and Britain have all denounced the Chinese government’s treatment of Ai, and have expressed their concern and disapproval about the most recent wave of activist disappearances in China. The U.S. government released a statement calling for Ai’s immediate release. In addition, William Hague, the British foreign secretary has released a statement reading “I call on the Chinese government to urgently clarify Ai’s situation and well-being, and hope that he will be released immediately.”

Human rights groups, friends and family of Ai have expressed concern, and they say that they believe that Ai is in danger of being tortured by the Chinese government.