China blocked one of its most famous contemporary artists from taking a flight to Hong Kong on Sunday and police later raided his Beijing studio, the man's assistant said.
The artist, Ai Weiwei, is an outspoken government critic and has been barred from going abroad before.
China has launched a massive crackdown on lawyers, writers and activists, arresting and detaining dozens since February when online calls for protests similar to those in the Middle East and North Africa began to circulate. Ai has been keeping an informal tally of those detentions on Twitter, where he has more than 70,000 followers.
The studio assistant, who asked not to be identified by name, said Ai was going through customs at the Beijing Capital International Airport early Sunday when two officials escorted him away, leaving a traveling companion to board the flight alone.
It was not clear whether the 53-year-old artist and architectural designer had been detained or why he was barred from taking the flight, the assistant said. Ai's cellphone could not be reached and airport police refused to comment.
Police later arrived at Ai's studio with a search warrant and took several staff members to a police station for questioning, said the assistant, who was among the group taken by police. A man who answered the phone at the police station said he would check on the case, then hung up the phone. Subsequent calls to the number rang unanswered.
Around two dozen uniformed and plainclothes police could be seen in and around Ai's studio Sunday afternoon. An Associated Press videographer was told by police to stop filming and leave the area.
Ai, an avant-garde artist who recently exhibited at the Tate Modern gallery in London, was stopped from boarding a flight to Seoul in December. That incident came shortly after he had been 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 had blocked him at the boarding gate and showed him a handwritten note that said he could cause damage to national security by leaving.
The son of one of China's most famous modern poets, Ai was 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.
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 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.
"The focus seemed to be on the fact that he had foreign employees," Klayman said by telephone from New York.
Associated Press videographers Isolda Morillo and David Wivell contributed to this report.
(This version CORRECTS in paragraph 8 that Ai was stopped from boarding flight to Seoul in December, not November)