France and Germany are leading calls for the release of Chinese artist Ai Weiwei, who remains missing more than 36 hours after his detention.

"I appeal to the Chinese government to urgently provide clarification, and I expect Ai Weiwei to be released immediately," the German foreign minister, Guido Westerwelle, said in a statement.

Earlier this week, Ai said that he was building a studio in Berlin, partially in response to the increasing pressure he faced in China.

French foreign ministry spokesman Bernard Valero said his country was very concerned and hoped the artist would be released as soon as possible, Agence France-Presse reported.

Ai, who created Tate Modern's Sunflower Seeds, was due to visit London next month for the unveiling of two major exhibits. A spokesman for the British embassy said the UK was monitoring Ai's case closely, along with those of other dissidents reportedly detained recently.

The artist's detention has highlighted the toughest crackdown on activists and dissidents in a decade, which has seen at least 24 people criminally detained, three more formally arrested for incitement to subversion and a dozen go missing.

Officers released the artist's wife and several assistants late last night, following questioning, but Ai and a friend remain uncontactable.

Assistants said that police removed documents and 30 computers and hard drives from his studio and home in north Beijing. They also searched other properties connected to the artist.

The scope of the police operation, and the length of time he has been missing, have increased friends' concern. Officials had also visited his studio three times in the week before his detention.

Ai's wife, Lu Qing, told the Associated Press that police had given no indication of her husband's whereabouts or why he was being held.

"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.

Beijing police have not answered queries about Ai or his friend Wen Tao, who was detained by police at a property near the artist's studio. Police searched Wen's home and removed a computer, a friend tweeted today, citing the 38-year-old's girlfriend.

Wen had previously worked as a reporter for state media, mostly covering sport.

"Like everyone who admires and respects the work of Ai Weiwei, we are dismayed and aggrieved by the news that [he] has been detained ... We deplore the raid on his studio and are working to establish contact with Ai Weiwei. We hope he is unharmed," said Gwyn Miles, the director of the Somerset House Trust, which will show one of his sculptures next month.

The Lisson Gallery in London is also due to hold a major exhibition of his work.

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

"There seems to be no reason whatsoever for his detention, other than that the authorities are trying to broadcast the message that China's time for open dissent has come to an end," said Donna Guest, Amnesty International's deputy director for the Asia-Pacific region.

"If even people like him are taken away, it gives a very bad sign to other human rights defenders and netizens," said Patrick Poon, executive secretary of the China Human Rights Lawyers Concern Group.

Five well-known human rights lawyers are among those missing since February. Poon said another one, Liu Zhengqing, was taken away from his home in Guangzhou on 24 March.

But to the surprise of friends and family, the artist Wu Yuren - known to some as "little Ai" because of his activism - has been released on parole despite the crackdown. He was detained for 10 months and stood trial for assaulting police, but his supporters believe the case was retaliation for his campaigning. His wife Karen Patterson said he was in good health but was still awaiting sentence and would not be giving interviews or commenting.