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Even before he assumed the presidency, Mr. Xi, who is also general secretary of China’s Communist Party central committee, pledged to go after “tigers” and “flies” — a reference to powerful leaders and lowly bureaucrats.

Xi Jinping goes out in public and calls for a crackdown on corruption, and he gets to run the country, while [anti-corruption activist] Liu Ping does that and she gets prosecuted. The hypocrisy is astonishing,” Ms. Richardson said.

Ms. Liu — along with fellow activists Wei Zhongping and Li Sihua — was put on trial on Monday in China’s eastern Jiangxi province on the charge of illegal assembly.

Some analysts are skeptical about how far Mr. Xi’s anti-corruption campaign will go.

“In a sense, corruption is the glue holding together the system. Providing the ability to accumulate illicit wealth is one way for the [Communist] Party to secure loyalty from its members,” said Edward Schwarck of the Royal United Services Institute for Defense and Security Studies in London.

“That is why Xi’s campaign can only go so far before it begins to threaten the foundations of party rule. The New Citizens' Movement has shone a light on the hypocrisy behind the anti-corruption campaign and is now being targeted as a consequence.”

Mr. Fu, who first got to know Mr. Guo in 2004 and helped his family come to the U.S. in 2009, said Mr. Guo himself was determined not to flee.

“He preferred to stay in China because he felt China needs him more than the U.S.,” Mr. Fu told The Washington Times.

Mr. Fu last spoke with Mr. Guo in July and said the activist had no inkling of his impending arrest.

“He sensed there was a major crackdown, but felt he was safe because he was doing everything as per the law,” Mr. Fu said.

Mrs. Zhang called on the Obama administration and Congress to press China to release Mr. Guo and other prisoners of conscience. She also asked that officials at the U.S. Embassy in Beijing and U.S. Consulate in Guangzhou visit Mr. Guo to ascertain his well-being.

Mrs. Zhang described her husband as an idealist who has been on the front lines of the struggle for democracy and freedom in China.

“That’s why he was singled out to be persecuted this way and he deserves our attention from the outside world,” she said.