Beijing, China (CNN) -- Police surrounded the studio of one of China's most famous artists Sunday after authorities reportedly detained him at a Beijing airport.
Artist Ai Weiwei and an assistant posted on Twitter that they were taken into custody while on their way to Hong Kong.
An assistant later posted that eight of the studio employees had been called in for questioning.
Neither the artist nor the assistant could be reached by phone and their whereabouts were unclear.
Authorities did not respond to repeated requests for comment from CNN.
Ai is an internationally recognized artist and an outspoken critic of the Chinese government.
He helped design the iconic Bird's Nest stadium for the 2008 Beijing Olympics, but later called for a boycott of the games because he believed China was using them as propaganda.
The artist has also accused the Chinese government of trying to silence dissidents.
"They crack down on everybody who has different opinions -- not even different opinions, just different attitudes," he told CNN last year. "Simply to have different opinions can cost (dissidents) their life; they can be put in jail, can be silenced and can disappear."
In the past Ai has noted that he has paid a price for his own dissent.
In January, the artist said authorities placed him under house arrest to stop him from attending an event he planned after officials announced that his new studio would be demolished.
"Just recently I heard the new studio I built in Shanghai will be demolished, all because of my activities," Ai said at the time. "So what is my activity? My activity is very simple, asking basic rights for people to freely express themselves and also to find a new structure, a new way of communicating. Because I'm an artist and this is what I do and I believe in that."
In the past several weeks, Chinese authorities have detained and arrested a number of lawyers and human rights activists amid calls for anti-government protests similar to those that have swept the Middle East.
CNN's Jo Kent contributed to this report.