As reported by Keith B. Richberg in the Washington Post, April 7, 2011:

The Chinese government on Thursday confirmed it is holding the missing artist Ai Weiwei, who has been incommunicado since police seized him Sunday at Beijing’s airport while other officers raided his art studio.
A foreign ministry spokesman said Ai, an outspoken critic of the government, was being investigated for “economic crimes” and warned other countries not to meddle in the case.
“China is a country ruled by law and will act according to law,” said Hong Lei, the foreign ministry spokesman, at a regularly scheduled press briefing. “We hope that the countries concerned will respect China’s decision.”
“This has nothing to do with human rights or freedom of expression,” Hong said, adding: “Other countries have no right to interfere.”
Reporters pressed the spokesman several times for more details on Ai’s detention, but he refused to say more. Hours later, in an odd development, the foreign ministry issued an official transcript of the Thursday briefing that deleted all the questions and answers about Ai. Typically, the briefing transcripts appear on the ministry Web site unaltered.
Xinhua, the state-run news agency, issued a brief, one-line report late Wednesday saying Ai was suspected of economic crimes. But that posting, too, was soon redacted from the Xinhua Web site. Some Internet users took a screen shot of the original Xinhua post before it was erased and displayed it on their Twitter accounts.
During the foreign ministry briefing, Hong did not elaborate on the charges against Ai. Chinese authorities sometimes use “economic crimes,” and specifically tax evasion charges, to try to silence dissenters.
Ai, a conceptual artist whose work has been displayed around the world, is the most prominent figure arrested so far in a sweeping security crackdown in China that has netted dozens of human rights lawyers, activists, writers and bloggers. The U.S. State Department, the European Union and various human rights groups have called for the release of Ai and all the detainees.
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Ai WeiweiSunflower Seeds, 2010. 100 million unique, individually handmade porcelain seeds, weighing over 100 metric tons and covering 1000 square meters of the Tate Modern's Turbine Hall. Per the Tate:
For Ai, sunflower seeds – a common Chinese street snack shared by friends – carry personal associations from the Cultural Revolution (1966-76). While individuals were stripped of personal freedom, propaganda images depicted Chairman Mao as the sun and the mass of people as sunflowers turning towards him. Yet Ai remembers the sharing of sunflower seeds as a gesture of human compassion, providing a space for pleasure, friendship and kindness during a time of extreme poverty, repression and uncertainty. 
Photos 1+ 2 by Marcus Leith & Andrew Dunkley, all images copyright Tate Photography. Via dezeen.