Chinese officials have rounded up dozens of Beijings's tiny coterie of activists and petitioners in case any dissident tries to approach President Obama, who arrived in the city today.
The arrests continued to gather momentum even as Mr Obama told an unprecedented question-and-answer session with Shanghai students that freedom of information and expression were vital for a stronger, more creative society.
Among those detained was Qi Zhiyong, a dissident who lost a leg during the crackdown on the student-led protests in Tiananamen Square in 1989. He said that he had been held for trying to organise a human rights seminar on November 9 in a Beijing park. He and fellow organisers had planned for the seminar to last until the end of President Obama's visit.
He had applied to police to stage a protest during Mr Obama's visit "to press him to pay attention to human rights in China, people's livelihoods and the relatives of jailed people, as he comes only to talk about climate change".
Mr Qi said he was being held in the Beijing suburbs and had been charged with unlawful assembly and disturbing the social order.
Mr Obama set foot on Beijing soil several hours ago after Air Force One touched down in the city. White House officials have said that the issue of human rights will be on the table when he sits down with his Chinese counterpart, Hu Jintao, in Beijing's Great Hall of the People tomorrow.
Faced with such pressing priorities as climate change, the value of China's currency, trade disagreements and how to deal with nuclear-armed North Korea, however, the fate of dissidents such as the detained writer and democracy proponent Liu Xiaobo is likely to be left on the backburner.
Also detained was the activist Zhao Lianhai, who attracted the attention of the authorities last year when he organised an online support group for parents of the thousands of children who fell sick after being fed tainted milk powder last year.
The activist group Human Rights in China said that Mr Zhao was handcuffed and taken from his home late on Friday night by police officers who searched his house and took away computers, a video recorder, a camera and an address book. When he refused to go with them, because the summons did not state a cause, the police filled in a summons for "provoking an incident".
Chen Jianfang, a petitioner from Shanghai who travelled to Beijing with 200 others, said that the group wanted to welcome Mr Obama and to draw his attention to human rights violations in China. She said that several dozen of her companions were rounded up by Beijing police when they arrived at the government's petition office today.
She said: "They are detaining people everywhere, even if they are only petitioning normally at the state petition office and are not holding any banners or shouting any slogans."
China's traditionally paranoid security authorities have a long history of detaining dissidents to prevent any encounters with visiting US leaders.
In February 1989 police stopped China's then most prominent dissident, the astrophysicist Fang Lizhi, as he and his wife were on theri way to a Texas-style barbecue dinner as guests of visiting President George Bush. They were stopped several blocks from the banquet and told that they were not on the guest list.