When President Bush arrived in Beijing last weekend, China greeted him with a red carpet, twin national anthems and President Hu Jintao's commitment to "raising the level of human rights enjoyed by the Chinese people."

But far from the cameras, China mobilized for Bush's visit in a very different way: by detaining, monitoring or silencing a vast array of government critics.

The campaign, quietly launched days before Bush's arrival, swept up activists and their relatives, lawyers, petitioners and dissidents, young and old, the prominent and the unknown. It was systematic and highly effective. An examination of how that process unfolded offers a window into the mechanics of authoritarianism, a primer on how China's regime blunts what it sees as the dangers of free expression and packages an image to the world.

The 73-year-old father of a Shanghai dissident found himself trailed by four plainclothes officers. Across town, a housing activist says she was forcibly driven to the city's outskirts and detained for four nights. Hundreds of miles north, some 30 petitioners to the government were rounded up as they headed to a church in Beijing. And deep in the Shandong countryside, a farmer with a politically sensitive lawsuit against local officials was taken into custody, where he remains.

There is no firm count of the people detained or silenced during Bush's visit, but rights advocates here and abroad conservatively estimate the total to be several hundred. Those detained are part of what Chinese regulations call the zhongdian renkou, or target population, a disparate list of men and women who have drawn the ire of the authoritarian regime.

"When I was in prison, I was kept in a small pen with a wall. Since leaving prison, I'm simply kept in a bigger pen that has no wall," said writer Liu Xiaobo, one of China's most prominent dissidents, speaking by phone while under house arrest in Beijing.

In interviews, five of those detained, monitored or confined said police identified Bush's visit as the cause. Others said they were given no reason, though they believe the timing and their experiences during previous high-profile visits explain why they were targeted. A few who remain under house arrest said they believe it was because of an ongoing visit by the United Nations special rapporteur on torture.

China has long silenced big-name democracy activists on important dates. But recently, as the state confronts a rising wave of social unrest across inland China, authorities are casting a noticeably wider net to include minor protesters, petitioners and grass-roots leaders.

"These are not the classic dissidents," said Robin Munro, a rights expert at the Hong Kong-based China Labor Bulletin. "These are local activists ... who are rising up from local movements against official corruption, persecution and arbitrary behavior of various kinds. From a state control point of view, the dissidents are probably less worrying than this new generation of grass-roots activists."

Hu Jia knew what to expect, so he ran. The 31-year-old Beijing AIDS and pro-democracy activist said he was confined to his home during the March visit by U.S. Secretary of State Condoleezza Rice. When he tried to leave, Hu alleges, police officers punched him six times in the face. So when he noticed two black sedans, of the style used by state security officers, parked outside his home earlier this month, he sneaked away and remains in hiding, sleeping in train stations, airport terminals and friends' homes.

"The police try to deprive us of our basic rights. They are like a state-sanctioned mafia. What they do is outside the law," he said in a phone interview.

The way China treats its people during interactions with the world is of growing urgency as Beijing prepares to host the 2008 Summer Olympics. Four years ago, in its bid for the Games, China touted the continued "development of democracy and the rule of law."

Nevertheless, the U.S. State Department and human-rights organizations have documented China's ongoing use of unofficial detention during politically important visits and anniversaries. In the case of Bush's trip, Rice told reporters that the United States protested the clampdown "quite vociferously."

Decision-making within China's security apparatus is opaque, so it can be difficult to know the official reason for an arrest. Asked about the recent detentions, a Foreign Ministry spokesman said most such reports are false but added that "it is necessary to take security measures" during a visit by a head of state.

Police in Beijing and Shanghai declined to comment. Foreign researchers who study China's handling of dissent say this type of clampdown typically originates with an official order that gives broad latitude to local police on who is targeted and how.

"The neighborhood police station will receive a circular that says something like, `In two weeks, there will be a visitor from this foreign country, and it is up to you to keep public order,'" said Nicolas Becquelin, a Hong Kong-based researcher for Human Rights in China. "The police will check their lists and then dispatch people to go pick up the relentless petitioners, or disconnect the phones, or post an officer in front of the door, or some other method."

For Xu Yongdao, the process began the day before Bush arrived, when the Shanghai retiree discovered four plainclothes officers following him when he went grocery shopping. Xu had attracted police scrutiny because he wrote an open letter to Bush appealing for help on behalf of his son, Xu Zhengqing, who was sentenced to three years in prison after repeatedly organizing protests. The father was followed around the clock until after Bush left China, his daughter-in-law said.

Housing activist Wang Liqing received more-aggressive attention. Two days before Bush's arrival, the 44-year-old Shanghai resident was returning home when plainclothes officers stopped her at the front gate. When she refused to go with them, Wang alleges, she was forced into a car, driven across town and carried by six men into a basement room of a motel. Wang says she was held for four days but not mistreated. She says she was monitored 24 hours a day, and her cell phone was disabled.

Saying something was what housing activist Tian Baocheng intended to do. He joined 30 or so other citizens critical of Shanghai housing policies on a trip to Beijing in the hope of sharing their stories with Bush's delegation. But as Tian and others set off Sunday morning to the church where Bush was worshiping in a gesture of support for religious freedom, they were intercepted by police, returned to Shanghai, interrogated and released, Tian said.

But not all detentions posed so clear a connection to Bush's visit. Police in Shandong province's rural Yinan County did not explain the timing when they came for 39-year-old farmer Liu Nai Chun the morning the U.S. president arrived.

Liu had been under pressure from local police for seven months, ever since he filed a lawsuit alleging that he was beaten by officers who were trying to seize his prized ox - part of what he said was local family-planning officials' attempt to pressure his sister to agree to a forced sterilization.

That campaign of forced sterilizations later became politically charged because Beijing harshly criticized the local government for breaking family-planning laws. County officials pleaded with Liu to drop his suit. They offered him a job and cash, but he refused to relent, according to Teng Biao, a Beijing-based scholar who has researched the family-planning controversy.

Officially, Liu was arrested on charges of riding his motorcycle without a license. Supporters, however, point out that few in Liu's remote village have ever thought to seek a motorcycle license. And besides, they say, police had been stationed outside his relatives' home for 48 hours before he arrived on foot and was arrested.

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