One of China's best-known artists, Ai Weiwei, was detained in Beijing on Sunday at the capital's airport, as police searched his studio, confiscated computers and questioned assistants.
Ai, who designed the Olympic Bird's Nest stadium and created the Sunflower Seeds installation for the Tate Modern in London, has been an outspoken critic of the government.
At least 23 people have been criminally detained, mostly for incitement to subversion or creating a disturbance, in recent weeks. Three more have been formally arrested and more than a dozen are missing, including high profile human rights lawyers.
A staff member told the BBC Chinese news service that officers had taken away eight of Ai's assistants and volunteers. A friend of the artist later tweeted that most had been released but that his wife Lu Qing and two employees remained out of contact.
Asked about Ai, an airport police spokesman said: "I do not have the obligation to tell you the information. You may have got your information wrong. Even if it is right, you have to go through certain procedures to make inquiries, not just make a phone call."
Last year Ai was placed under house arrest after announcing a party to mark the forced demolition of his studio in Shanghai. In December he was prevented from leaving the country, amid government fears that people would attend the Nobel Peace Prize ceremony for the jailed writer Liu Xiaobo.
British Foreign Secretary William Hague said, "I am very concerned by reports that Ai Weiwei has been detained by the Chinese authorities. I call on the Chinese government to urgently clarify Ai's situation and well-being, and hope he will be released immediately."
Curators said a large-scale public art project by Ai will occupy the Pulitzer fountain outside the Plaza Hotel in New York, starting early next month, even if the artist is unable to be present.