Ai Weiwei, China's best-known artist, remains missing more than a day after he was detained. Police have confiscated dozens of items from his studio.

Officers released his wife and several assistants late last night, following questioning, but Ai and a friend remain uncontactable. Assistants said that police removed more than 30 computers and hard drives from his studio and home in north Beijing on Sunday, as well as notebooks and documents. They also searched at least two more properties connected to the artist.

The scope of the police operation, and the fact that Ai was detained at Beijing airport on Sunday morning – not turned away from his flight, as had happened before – has increased the concern of friends. Officials had also visited his studio three times in the week before his detention.

"There is no news of him so far," his wife, Lu Qing, told the Associated Press.

"They asked me about Ai Weiwei's work and the articles he posted online ... 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 police gave no indication of her husband's whereabouts or why he was being held. She added that his mother, who is in her 80s, was very anxious about her son's fate.

Beijing police told the Guardian they knew nothing of Ai or the other missing man, Wen Tao. An airport police spokesman said he had no obligation to give out information.

Although the 53-year-old artist has repeatedly clashed with authorities owing to his outspoken criticism of the government, he was thought to enjoy greater latitude than most thanks to his father's status as a revered poet and his own high international profile. He also helped to design the Olympic Bird's Nest stadium.

Ai created last year's Sunflower Seeds installation at the Tate Modern turbine hall in London. His exhibition at the Lisson Gallery, also in London, is due to open next month, shortly after his recreation of a Chinese zodiac sculpture is unveiled at the courtyard in Somerset House.

In an interview last year, asked about the possibility of retribution from the authorities, he told the Guardian: "I have to deal with it, but not to prepare for it, because it is a kind of stupidity. If you prepare for it too much, you become a part of it."

His detention comes amid a widespread crackdown on activists and dissidents in China, which has seen more than 20 people criminally detained, three formally arrested for incitement to subversion and a dozen go missing.

"It is getting worse and worse. Ai Weiwei is a very influential figure ... [if] even people like him are taken away, it gives a very bad sign to other human rights defenders and netizens [socially concerned internet users]," said Patrick Poon, executive secretary of the China Human Rights Lawyers Concern Group.

Five human rights lawyers are among those missing since February and Poon said it had now emerged that another one, Liu Zhengqing, was taken away on 24 March.

Liu had been travelling for several weeks and friends lost contact with him when he returned to his home in Guangzhou. Poon said it was unclear why Liu was held, but that it might be related to his agreement to represent one of the lawyers who had already gone missing.