|US, France, Germany and Britain have joined Amnesty International in calling for Ai Weiwei's release [AFP]
The wife of a missing Chinese artist says Chinese police are questioning Ai Weiwei's friends and collaborators amid international concern over his apparent detention by authorities.
Ai, an outspoken government critic, has not been seen since apparently being taken into custody after he was barred from boarding a Hong Kong-bound flight at a Beijing airport on Sunday morning.
His disappearance comes as Chinese security services carry out a massive crackdown on lawyers, writers and activists following online calls for protests in China similar to those in the Middle East and North Africa.
Dozens have recently been taken into custody with little word from authorities about where they are being held, who is holding them or the crimes that they are suspected of having committed.
Police searched Ai's home and studio shortly after his detention and removed computers and other items.
Ai's wife, Lu Qing, said friends and family were asking police for information about his whereabouts and that of an assistant, Wen Tao, who was detained along with him. So far, they had learned nothing, Lu said.
"I am very worried," Lu told The Associated Press news agency by telephone. "I felt something terrible was going to happen when they came to search the house and took away all those things."
Lu said that friends of Ai and people who have collaborated with him, are being contacted for questioning.
Police appear to be working their way down a detailed list of both Chinese citizens and foreigners associated with Ai, said Alison Klayman, an American filmmaker who has been working on a documentary about the artist
A Beijing police spokesman, speaking on condition of anonymity, said he had no information on Ai's case.
Ai is the son of one of China's most famous modern poets, and that stature, led many to believe he was protected from serious attack or formal arrest.
Among China's best-known artists internationally, Ai recently exhibited at the Tate Modern gallery in London.
His career spans protests for artistic freedom in 1979, provocative works in the 1990s, and a hand in designing the Bird's Nest stadium for the 2008 Beijing Olympics.
He was stopped from boarding a flight to Seoul in December, shortly after being invited to attend the Nobel Peace Prize ceremony in Oslo, Norway, honoring Liu Xiaobo, jailed Chinese dissident.
Liu is serving an 11-year sentence for subversion.
Ai said at the time that police blocked him at the boarding gate and showed him a handwritten note that said he could cause damage to national security by leaving.
On Monday, Mark Toner, US State Department spokesman, called for Ai's immediate release.
He said that Washington was "deeply concerned by the trend of forced disappearances, extralegal detentions, arrests and convictions of human rights activists for exercising their internationally recognised human right for freedom of expression.''
And William Hague, British foreign secretary, also expressed concern and said London was committed to engaging China on human rights issues.
On Tuesday, the European Union delegation in Beijing called on Chinese authorities "to refrain from using arbitrary detention under any circumstances".
Chinese activists are also increasingly alarmed about Ai's extended detention, and supporters in China and abroad launched their own online drive urging authorities to free him.
The online petition to "free Ai Weiwei" was launched on Twitition, a Twitter microblog site, which China's wall of Internet censorship stops most Chinese from seeing.
Enquiries about Ai on China's most popular homegrown microblogging site, Sina.com's "Weibo," have also been blocked.