Ai Weiwei remains in detention by Beijing authorities for a second day with no reason provided, according to the Chinese artist and activist's wife.

The contemporary artist and outspoken critic of the Chinese government had been preparing to board a flight to Hong Kong with assistants on Sunday when he was taken into custody.

Several of his assistants were detained, questioned and released as of Monday, with one ordered to continue on her travels to Hong Kong.

Ai's wife, Lu Qing, said police raided the couple's home and artist studio in Beijing Sunday night. There, they searched and removed items, including documents and computer components. Lu says she was also interrogated by police.

"They took the computer, computer disks and other materials. They refused to say why the search warrant was issued or why Ai Weiwei was taken away," she told Agence France-Presse.

His family has received no word about Ai's whereabouts or his status.

A producer and crew from Dan Rather Reports say that Ai spoke of his fear of being arrested in a recent interview at his Beijing studio. That interview is to air on HDNet on Tuesday.

Several parties have already spoken out against his detainment, including the press freedom group Reporters Without Borders as well as French and German officials.

"The Chinese government is stepping up its harassment of the remaining prominent dissidents and is trying to silence all of its critics," Reporters Without Borders said in a statement on Sunday.

"We urge the international community to react firmly to the arrests of bloggers and cyber-dissidents that are taking place at an unprecedented rate and to quickly denounce the increased harassment of foreign reporters in China which, we fear, is forcing many of them to censor themselves."

"We are very concerned about the fate of the militant artist Ai Weiwei and we are following his situation and that of his family very closely," said Bernard Valero, a spokesman for France's foreign ministry. "We hope he will be released as soon as possible."

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

Fierce criticism

Ai, 53, has been a fierce critic of Chinese leaders on many issues, including corruption and human rights violations.

He has developed an international reputation for high-profile cultural work over the past decade, including his collaboration on the acclaimed "Bird's Nest" stadium built for the Beijing Olympics in 2008 and the popular Sunflower Seeds art installation at London's Tate Modern in 2010.

His activist projects have also drawn attention, though they typically earn him censure by the government. This politically and socially motivated work has included making a film about a fellow Chinese activist denied re-entry into Shanghai and creating an online series exploring the death toll from the Sichuan earthquake, which killed thousands of schoolchildren.

His blogs and artistic work posted online have repeatedly been shut down and he has suffered beatings, such as the one just prior to his testimony on behalf of another activist researching the earthquake.

In January, his newly built Shanghai art studio — which he was lured by city officials to construct in a developing area — was razed.

Last week, Ai expressed a desire to set up a studio in Germany to thwart continued efforts to silence him, prevent his travel and his exhibitions.

"It's very discouraging what's happening here and if I want to continue to develop my work, I have to find a base," he told Agence France-Presse.

Ai's detention this week comes amid a recent crackdown on writers, lawyers, activists and government critics. Ai had been keeping an informal tally of those detained via his Twitter account.