China confirmed Thursday that it has detained a renowned artist who had been missing for four days, but insisted his case involves economic crimes and not human rights.

Ai Weiwei, an internationally famed avant-garde artist who is also an outspoken government critic, was last seen early Sunday in police custody after he was barred from boarding a flight to Hong Kong at a Beijing airport.

Foreign Ministry spokesman Hong Lei told a regular news conference that police were investigating Ai for unspecified economic crimes.

"It has nothing to do with human rights or freedom of expression," Hong said.

"The public security authorities are conducting an investigation according to law. China is a country under the rule of law and relevant authorities will work according to law," he said, giving no other details.

Chinese authorities sometimes try to silence critics by accusing them of tax violations or other non-political crimes. Beijing police refused to comment on Ai's case.

Ai helped create Bird's Nest stadium

Born 1957 in Beijing, the artist has been highly vocal about human rights issues in his country. Ai helped create Beijing's Olympic Bird's Nest stadium and he recently exhibited an installation of sunflower seeds at London's Tate Modern art gallery.

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A supporter of prominent Chinese artist Ai Weiwei holds a picture of him at a protest to try to stop the demolition of his studio by the government in Shanghai Nov. 7, 2010. (Carlos Baria/Reuters)

He is the most prominent target so far in China's massive crackdown on dozens of lawyers, writers and activists following online calls for protests similar to those in the Middle East and North Africa. No protests have occurred in China.

Several countries, including the U.S., Britain, and Germany, have raised concerns about Ai's detention. However, Hong dismissed their remarks, saying, "Other countries have no right to interfere."

Ai has had past run-ins with authorities, but his wife said the current situation was worse.

His detention has sent a chill through the activist community and prompted many to call for his release online through Twitter messages or blog postings.

'Political persecution'

In previous cases involving economic crimes that others saw as political persecution, Zhao Yan, a news assistant for The New York Times, was jailed for three years in 2007 on charges of financial fraud. Xu Zhiyong, an outspoken lawyer, was investigated for alleged tax evasion in 2009 but later released.

On Wednesday, U.S. Ambassador Jon Huntsman mentioned Ai among other activists who "challenge the Chinese government to serve the public in all cases and at all times."

"The United States will never stop supporting human rights because we believe in the fundamental struggle for human dignity and justice wherever it may occur,"  Huntsman, who leaves his post later this month, said in a speech in Shanghai.