| Ai has had regular run-ins with authorities over the past few years [EPA]
A prominent Chinese artist has been missing since police prevented him boarding a flight on Sunday afternoon, according to an assistant.
Ai Weiwei, who was prevented from flying to Hong Kong, is among the most high profile critics of the China's ruling Communist Party. He has exhibited at London's Tate Modern gallery and speaks regularly to Western media
An assistant at Ai's Beijing studio told the Reuters news agency she was among eight people connected to the artist who were taken for questioning by police and later released on Sunday. Ai's wife and driver were also detained.
All had been released, but the assistant said they'd had "no information whatsoever" about Ai's whereabouts.
His apparent detention would add to a lengthening list of dissidents held in the past two months and drew immediate concern from numerous human rights groups.
The clampdown followed anonymous online calls which emerged in February for protests each Sunday around the country to demand political change in China, aimed at emulating those that have rocked the Arab world.
Ai has the highest international profile of the scores of critics detained in the past two months and had been keeping an informal tally of those detentions on Twitter, where he has more than 70,000 followers.
Speaking to Reuters, Ai's assistant, who asked not to be named, said she didn't know why he had been prevented from boarding the flight or the reason for his apparent detention.
"We don't know where he is," she said. "We hope that he can be released as soon as possible."
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.
A frequent critic of China's Communist Party leaders, Ai, who investigated school collapses in the massive 2008 earthquake in the southwestern province of Sichuan, has had regular run-ins with authorities over the past few years.
However, the studio assistant said she had no way of knowing whether this time was more serious.
Ai is one of China's most famous contemporary artists. 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.
His public comments, activities and art are some of the loudest, most flagrantly defiant forms of speech in China today, where government controls on the Internet, and traditional media constrain civil society.
Ai has never been formally arrested, despite his many brushes with the law.
Last year, he 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.
He was also placed under house arrest last year after an argument with the government over the demolition of his studio in Shanghai.