Tensions grow between Beijing and Washington over political activist who fled house arrest and is said to be in US embassy

Concern is growing for relatives and supporters of Chen Guangcheng, the blind legal activist who fled from 19 months of house arrest in a Chinese village in Shandong province and is said to be sheltering inside the American embassy in Beijing. Police have detained four family members and two of those who aided his escape, a rights group said.

The US has declined to comment on Chen's whereabouts, but friends have said that he is at the embassy, although he had told them he did not wish to seek asylum. His case threatens to overshadow the arrival of US secretary of state Hillary Clinton and treasury secretary Timothy Geithner for bilateral talks in Beijing this week.

Chen, a self-taught legal activist, fell foul of authorities for exposing a mass campaign of forced sterilisations and abortions in his area. He was jailed for four years on what supporters said were trumped-up charges and put under house arrest following his release in September 2010. He and his wife have not even been able to meet neighbours or use the telephone or internet. At one stage their young daughter was prevented from going to school.

In a video appeal to Chinese premier Wen Jiabao, released as news of his escape broke on Friday, Chen said that he feared "insane retribution" against his family, who remain in Dongshigu village, Linyi prefecture, in the eastern province of Shandong.

The situation of his wife, mother and six-year-old daughter – who have also been living under house arrest – is unknown. But the Chinese Human Rights Defenders (CHRD) network said they believed four relatives were already in custody. Chen's older brother, Chen Guangfu, and nephew, Chen Kegui, were detained after guards broke into their home when they learned of the activist's escape. Chen Kegui said he used a knife to defend himself and his mother. Chen's cousin, Chen Guangcun, and his cousin's son, Chen Hua, were also taken into custody, CHRD added.

Activists He Peirong and Gao Yushan, who aided Chen's escape, are also thought to be detained. Zeng Jinyan, the wife of dissident Hu Jia – who met Chen after his flight to the capital – said he had been summoned by police for questioning on Saturday afternoon. Shortly afterwards, she too went to the police station with their young daughter.

"We're very concerned that authorities appear to have launched a round of retaliation against Chen's family, relatives and activists who assisted Chen Guangcheng's flight for freedom," said Renee Xia, international director of the CHRD.

The escape comes as the Chinese government is already coping with the fall-out from the scandal surrounding ousted leader Bo Xilai, and preparing for a once-a-decade transition of power. It could also add to tensions with the US.

"Chen is under US protection and high-level talks are under way between US and Chinese officials regarding his status," said a statement from ChinaAid, which has long pressed Chen's case, citing a source close to the situation.

Bob Fu of ChinaAid added: "Because of Chen's wide popularity, the Obama administration must stand firmly with him or risk losing credibility as a defender of freedom and the rule of law."

Asked if this week's meetings could be cancelled or postponed, the Chinese vice-foreign minister Cui Tiankai told reporters: "I don't know why you'd ask the question. Our holding this briefing today shows that the strategic economic dialogue will take place as scheduled." He added that he had "no information" on Chen.

The only precedent for Chen's case is thought to be that of dissident Fang Lizhi, who took refuge in the US embassy in 1989 following the bloody crackdown on pro-democracy protests. He stayed for more than a year before Beijing allowed him to leave for medical treatment in the US after protracted negotiations.

But Hu Jia said on Saturday that Chen had said he "wanted to stay and fight, and not request asylum". That may also reflect the fact that his family are still trapped in Shandong.

Nicholas Bequelin of Human Rights Watch added: "If he is in the embassy, it's a very tricky situation. I can't see what meaningful guarantees China could give the US government. They have always denied he was suffering persecution.

"It's as big a problem as China wants to make it ... This is not the China of 1989. He could spend five years in the embassy."

Bequelin said Beijing would be concerned not only about immediate embarrassment but also about the prospect of other dissidents who might attempt to follow suit.

"It is the Shandong government that has bungled this by turning a small matter into an international affair," said Li Datong, an independent political commentator. "It is certainly a loss of face for the central government, especially so if a Chinese citizen has gone to another government to seek protection."

Sympathisers have sought to evade censors by using a series of indirect references to discuss Chen's case online. But by yesterday afternoon even searches for "blind person" and "embassy" were blocked on Sina's popular microblog service. One scholar, Zhu Dake, posted a fable about a mole who escaped from a pack of wolves with the help of mice.

"The mole's older and younger brothers, his mother and his baby still lived in the burrow. They became the hostages of the wolves ... The mice in the walls and the mice in the field … couldn't decide whether the [mole's] escape was a victory, or whether it was just the beginning of more hardship," he wrote.