Story highlights Sentence is "retribution," the State Dept. says

He was arrested last summer after three months under house arrest

Xu's lawyer says the guilty verdict was expected and that his client will appeal

An Amnesty official calls the conviction "shameful, but sadly predictable"

Xu Zhiyong, a high-profile Chinese activist accused of organizing protests against official corruption, has been convicted and sentenced to four years in prison, a Beijing court announced Sunday.

The U.S. State Department immediately released a statement saying the sentence was "retribution for his public campaign to expose corruption."

Chinese prosecutors alleged that Xu Zhiyong "gathered a crowd to disturb public order" after a series of small protests. The demonstrators unfurled banners in Beijing calling for officials to publicly declare their assets, said Xu's lawyer, Zhang Qingfang.

Zhang told CNN before Sunday's announcement from Beijing No. 1 Intermediate People's Court that they expected a guilty verdict and heavy sentence because of the case's political nature.

He said they would "definitely appeal."

The trial of Xu -- who founded the New Citizens Movement, which calls for official transparency and rule of law -- attracted widespread attention and authorities have come down hard on international media covering the case.

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A CNN crew recently was prevented from approaching and filming the courthouse. CNN's Beijing correspondent, David McKenzie, was kicked, pushed and punched by Chinese security before being forced into a nearby van and driven away.

Roseann Rife, an official with rights group Amnesty International, called the verdict "shameful, but sadly predictable."

"The Chinese authorities have once again opted for the rule of fear over the rule of law," Rife said in a statement. "At best, the injustice of prosecuting Xu Zhiyong is hypocrisy of the highest order."

Lawyer: Xu removed from court

A former law lecturer at the Beijing University of Posts and Telecommunications, Xu has a reputation for taking on groundbreaking legal causes.

He has campaigned for death row inmates and families effected by a poisoned milk formula scandal in 2009.

On July 16, Xu was arrested after spending more than three months under house arrest at his Beijing home.

During his trial, Xu tried to read a prepared statement highlighting the goals of the movement and declaring his innocence but was interrupted by the judge when he spoke about wanting top leaders to be transparent about their assets, his lawyer said.

"Calling on officials to reveal their assets was our effort to promote the country's anti-graft system," Xu said, according to his lawyer.

"More than 137 countries and regions in the world have established such systems, why can't China? What are those 'people's servants' afraid of?"

Authorities removed Xu from the courtroom after he made those remarks, his lawyer said.

Xu's name, 'constitutional rule' blocked on search engines

Well before last week's trial, Xu had become an international figure as activists and others demanded his release.

Outgoing U.S. Ambassador to China Gary Locke issued a statement saying he was "deeply concerned that ... the prosecution of (Xu and) other good-governance advocates advocates are retribution for their public campaigns to expose official corruption and for the peaceful expression of their views."

Searches for Xu's name and "constitutional rule" were blocked on Chinese microblogs and search engines around the time of his trial.

It began amid new and potentially embarrassing revelations about the wealth of China's elite.

The International Consortium of Investigative Journalists reported that close relatives of China's top leaders own offshore companies in international tax havens.

The report said that confidential files showed that President Xi Jinping's brother-in-law and the son of former Premier Wen Jiabao had set up companies in the British Virgin Islands.

While not illegal, the findings shed light on the wealth of the country's leaders at a time when President Xi is intensifying his campaign against high-level corruption, the report said.