BEIJING—The U.S., Britain, France, Germany and the European Union called for the release of Ai Weiwei, one of China's most famous artists, as his detention by Chinese authorities raised fears among his supporters that he could be charged with subversion or held indefinitely in extra-judicial custody as dozens of other activists have been over a six-week crackdown on dissent.

Mr. Ai, an outspoken critic of the government who has more than 70,000 followers on Twitter, has been out of contact since Chinese officials prevented him from boarding a flight from Beijing to Hong Kong on Sunday morning and then led him away, according to several of his friends and assistants.

Liu Xiaoyuan, Mr Ai's lawyer, late Monday criticized the Chinese media on Twitter for not reporting the artist's disappearance. "Ai Weiwei has been taken away, and his studio has been searched," he wrote. "Whether Ai Weiwei is right or wrong, this is still really big news, a really hot topic. I never thought, never thought, that the domestic media would actually lose the power of speech, and act both deaf and dumb. Sad, really sad."

Mark Toner, a spokesman for the U.S. State Department said the U.S. government was "deeply concerned by the trend of forced disappearances, extralegal detentions, arrests, and convictions of rights activists for exercising their internationally recognized human rights, including freedom of expression and movement".

"The detention of artist and activist Ai Weiwei is inconsistent with the fundamental freedoms and human rights of all Chinese citizens, including China's commitments under the Universal Declaration of Human Rights, and we urge the Chinese Government to release him immediately."