One of China's most famous contemporary artists has been missing for more than a day after he was blocked from leaving the country and police raided his home, his wife said Monday.

The disappearance of artist Ai Weiwei comes as China carries out a massive crackdown on lawyers, writers and activists, arresting and detaining dozens since online calls for protests similar to those in the Middle East and North Africa began to circulate in February. No public protests have emerged.

Ai, an outspoken government critic, has been keeping an informal tally of those detentions on Twitter, where he has more than 70,000 followers. Ai, who has been barred from going abroad before, was stopped while preparing to board a flight to Hong Kong on Sunday. Police later raided his Beijing home and studio.

"There is no news of him so far," said Ai's wife, Lu Qing. Lu said she was interrogated Sunday night by Beijing city police, who searched the couple's home and took away items, including documents, computers and hard drives.

"They asked me about Ai Weiwei's work and the articles he posted online," Lu told The Associated Press. "I told them that everything that Ai did was very public, and if they wanted to know his opinions and work they could just look at the Internet."

She said a group of office employees who were detained when Ai's studio was searched had been released.

Lu said police gave no hints as to where Ai was, why he was being detained or how long he would be held. She said Ai's mother, who is in her 80s, was very anxious about her son's fate.

A question faxed to police Monday asking about Ai's situation was not immediately answered.

The son of one of China's most famous modern poets, Ai's stature led many to believe he was protected from serious attack or formal arrest. He had been courted by the communist government as a cultural ambassador before his advocacy on behalf of social activists apparently made him a target of Chinese authorities.

An assistant, who did not want to be identified by name because of the sensitivity of the incident, said Monday that there had been no word from Ai since he was escorted away by two officials while going through customs at the Beijing Capital International Airport early Sunday.

It was not clear why the 53-year-old artist and architectural designer was barred from leaving or who was now holding him.

Ai, an avant-garde artist, is perhaps China's best known artist internationally and recently exhibited at the Tate Modern gallery in London. 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 jailed Chinese dissident Liu Xiaobo. 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.

Known for his distinctive scraggly beard and stocky frame, Ai was a consultant for the futuristic Bird's Nest stadium at the Beijing Olympics before souring on the event. He was later beaten and detained while attempting to attend the trial of an advocate for victims of the devastating 2008 earthquake in the southwestern city of Chengdu.

Alison Klayman, an American filmmaker who has been working on a documentary about Ai for more than two years, said by telephone from New York that Beijing police visited Ai's studio three times in the past week, checking the passports and identification of Chinese and foreign assistants working there and some visiting architecture students from Europe.

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Associated Press writer Scott McDonald contributed to this report.

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Online: http://www.aiweiweifilm.org/en/