Two Chinese rights activists who sought a meeting with U.S. President Barack Obama hours before he left Beijing said on Friday they were briefly detained by police, and one said Washington's sway over rights in China had eroded.

Jiang Tianyong and Fan Yafeng said they were among several critics of China's restrictions on human rights who gathered near the U.S. Embassy in the Chinese capital on Wednesday, hoping to meet Obama before he left that night for Seoul.

Most of the group left as it became clear there be no chance of such a meeting, but Jiang and Fan said they stayed until plain clothes security police took them away for a few hours of "chats" late into the night.

"We had wanted to discuss (with Obama) the deterioration of religious freedom in China, as well as deteriorating treatment of other rights defenders," said Fan, a legal scholar who recently lost his job at the Chinese Academy of Social Sciences.

Fan is a Christian active in informal "house" churches frowned on and often closed by China's authorities, who want to limit worship to state-approved churches.

Jiang has defended other Chinese rights activists and volunteered legal aid to Tibetans arrested after demonstrations in 2008. He was one of almost two dozen lawyers whose licences to practise law were not renewed in May 2009.

On Thursday, a day after Obama left, Zhou Yongjun, a student leader of China's 1989 pro-democracy movement with residential rights in the United States, went on trial in China on fraud charges.

Fan said China's success in restricting the television broadcast of Obama's meeting to Shanghai itself, and avoiding any reporters' questions at the joint appearance with President Hu Jintao, suggested that U.S. sway over China had eroded.

"Now on the Chinese mainland and internationally, it's widely believed that Obama's visit to China was a big failure," he said.

"It didn't match Obama's status as a Nobel Prize winner," he said. "With the United States suffering a financial crisis, clearly this superpower has lost its direction."

Jiang said he was also detained for much of Thursday by officers who confronted him as he tried to take his daughter to school.

During his visit, Obama used the "town hall" meeting with Chinese students in Shanghai and a joint press appearance in Beijing with Hu to speak out broadly for human rights and the free flow of ideas.

But like previous visiting presidents, Obama avoided directly raising specific criticisms and cases concerning China in public.

Obama's predecessor, George W. Bush, met Chinese dissidents and Christian activists in the White House.