Beijing  - Chinese police have detained or placed under house arrest dozens of democracy and rights activists ahead of US President Barack Obama's first visit to the country, a rights group said Monday.

"The authorities appear concerned that these vocal critics will attempt to meet with President Obama, visiting US officials, or foreign journalists covering the visit while the president is in Shanghai and Beijing," the Hong Kong-based Chinese Human Rights Defenders (CHRD) said in a statement.

The roundup follows the standard practice of the ruling Communist Party to control well-known dissidents, activists, petitioners and members of illegal religious groups before important state events.

Beijing-based democracy activist and church member Qi Zhiyong told the German Press Agency dpa that police had kept him under house arrest since November 9.

CHRD listed several more activists who were either placed under "soft detention" at home, taken into police custody or forced to stay temporarily at government-run hostels away from their home areas.

Police warned other activists outside Beijing and Shanghai not to travel to the two cities during Obama's visit, the group said.

CHRD said the roundup reflected a "troubling pattern of arbitrary detention, intimidation, and repression of free speech seen during times in which global attention is focused on China."

It urged Obama to raise the issue and ask Chinese leaders to release the detained activists.

"While the government touts its 'future leaders' in letting President Obama meet with a select few students in Shanghai, it is silencing those true leaders who speak out for justice, human rights, and the rule of law," CHRD said.

The group was referring to Obama's "town hall" meeting with some 500 mainly young people on Monday in Shanghai, at which the US president promoted "universal rights" in broad terms without mentioning China's record on human rights and democracy.

Beijing-based activist Ding Zilin wrote an open letter appealing to Obama to press China to release dissident writer Liu Xiaobo, who was detained for organizing Charter '08 for democratic reform in China, Hong Kong-based newspaper Ming Pao reported.

Ding, who also signed Charter '08, could not be contacted by telephone on Monday.

She is a retired professor who helped found the Tiananmen Mothers group after her 17-year-old son was killed in the military crackdown that ended student-led anti-government protests in June 1989.

Other Charter '08 signatories were reportedly among those detained before Obama's visit, including Yao Lifa, a rights activist in the central province of Hubei.

Hu Shigen, a dissident who was previously imprisoned for organizing an opposition democratic party, was held under house in Beijing arrest since November 12, CHRD said.

It said Beijing-based Li Hai, a student activist in the 1989 movement, disappeared the same day, while police took online activist Liu Di for an enforced "vacation" in suburban Beijing on November 14.

CHRD, London-based Amnesty International, New York-based Human Rights Watch and other groups have urged Obama to press Chinese leaders to improve the country's human rights record.

"In the nine months since you took office, the trend on human rights in China has been distinctly negative," Human Rights Watch said in its open letter to Obama.  (dpa)