The United States, France, Germany and Britain have all said they're concerned about Ai's well-being. He was arrested Sunday at Beijing Airport while boarding a plane to Hong Kong. Shortly afterward, police raided Ai's studio in Beijing, confiscating computers and temporarily detaining his wife, Lu Qing, as well as several of his aides.
While Lu and the aides were released later Sunday, the Chinese government has yet to reveal Ai's whereabouts or level charges against him. The 53-year-old has been missing for more than two days.
The strongest appeals for his release came from Germany, where Foreign Minister Guido Westerwelle, who had just returned from a trip to China, called on Beijing to "to urgently provide clarification" over the artist's fate.
"I expect Ai Weiwei to be released immediately," he said in a statement.
Ai last month told German media he planned to open a second studio in Berlin, saying working in China became increasingly unbearable. On April 29, the internationally acclaimed artist was to open a new exhibition in the German capital.
His arrest is the latest step of a major crackdown on political dissent in China following pro-democracy uprisings in the Middle East and North Africa.
Eager to prevent similar unrest at home, Chinese security forces during the past weeks have detained and harassed dozens of regime critics, among them writers, lawyers and artists.
The EU representative in China, Markus Eder, in a Tuesday statement expressed concern over this "increasing use of arbitrary detention" against regime critics.
Famous to a popular audience for his help in designing the so-called Bird's Nest, the main stadium used in the 2008 Beijing Olympic Games, Ai has long been an art star popular with Western collectors.
Over the past years, he has developed into the best-selling living Chinese artist, with shows in every major Western metropolitan. Ai's latest exhibition runs through May 2 at the Tate Modern gallery in London, where his millions of tiny porcelain objects that look like sunflower seeds became an instant hit with visitors.
Besides creating memorable works of art, Ai has always been an outspoken and fierce critic of the Beijing regime. He drew attention to government-neglected victims of the 2008 earthquake in China and has shown solidarity with detained colleagues. Via Twitter, where he has more than 74,000 followers, he often attacked the regime for its crackdown on human rights.
While Ai has been under house arrest several times, his supporters had hoped that Ai's global fame would shield him from a longer period in jail.