BEIJING (REUTERS) - China decried what it said was interference in its internal affairs on Friday after both the United States and European Union voiced concern over the detention of a high-profile ethnic Uighur academic from the restive western region of Xinjiang.

Police in Beijing on Wednesday seized Ilham Tohti, a prominent economist who has championed the rights of the Muslim Uighur community in Xinjiang, at his home and his whereabouts were unknown, his wife and close friend told Reuters.

"Ilham is suspected of breaking the law. Currently, China's relevant departments are handling the case according to law," Chinese Foreign Ministry spokesman Hong Lei told a daily news briefing. "China's law is sacred and inviolable," he added. "We oppose any country or party using human rights as a pretext to criticise another country's normal law enforcement and interfere in its internal politics and judicial sovereignty."

The U.S. State Department on Thursday called on China to account for the whereabouts of Prof Tohti and at least six of his students who were also taken into custody and guarantee their rights, including freedom of expression.

Speaking to reporters in Beijing on Friday, Markus Ederer, the European Union Ambassador to China, said he was also concerned.

"We call on the authorities to clarify the charges which have not been made known and to clarify his whereabouts to his family," Mr Ederer said.

"I have called on the authorities to treat him in line with Chinese legislation. Substantiate the charges, which so far has not happened... If the charges cannot be substantiated, release him, and I think that is a general line which applies to all individuals in that situation."

Prof Tohti's detention is the latest indication of the government's increasing hardline stance on dissent surrounding Xinjiang, where violent riots in the past year have killed at least 91 people.

Rights activists say Prof Tohti has challenged the government's version of several incidents involving Uighurs, including what Beijing says is its first major suicide attack involving two men from Xinjiang in Beijing's Tiananmen Square, by pointing out inconsistencies in the official accounts.

Prof Tohti, an economics professor at Beijing's Minzu University which specialises in ethnic minority studies, told Reuters in November that state security agents had physically threatened him for speaking to foreign reporters.

His wife, Guzailai Nu'er, said she has not been given information about her husband. Aside from taking her children to school, at which times she is followed, she is prevented from leaving her home by dozens of police posted outside.

"If I wanted to leave I couldn't. I can't go out and I cannot get online," she told Reuters by telephone.

Many Uighurs, who speak a Turkic language, chafe at restrictions on their culture and religion, though the Chinese government insists it grants them broad freedoms.

China has blamed some of the violence on Islamist militants, but rights groups and exiles say China exaggerates the threat to justify its firm grip on energy-rich Xinjiang, which abuts Central Asia, Afghanistan, Pakistan and India.