Human rights groups expressed scepticism about the claims, saying the authorities were using the Olympic threat to crack down on innocent victims and peaceful critics of Chinese communist rule.

The state-run Xinhua news agency also reported today that 82 people have been detained in the predominantly Muslim region of Xinjiang for planning terrorist attacks during next month's Olympics.

Chen Zhuangwei, the police chief of Urumqi - the provincial capital of the far western region - was quoted as saying his forces had also detained "66 gang members of the 'three evil forces' of terrorism, separatism and extremism, and destroyed 41 training bases of 'holy war' from January to June". Few details were provided.

On Tuesday, police shot and killed five people in Xinjiang who were accused of similar crimes, although they were armed only with knives. In April, the authorities said they had uncovered a plot to kidnap athletes and carry out suicide attacks. State media early reported that police foiled an attempt to hijack a plane.

Overseas Uighur groups deny the allegations. Human Rights Watch said it was sceptical.

"The Chinese government has not provided any independent evidence to back these claims. All it has presented are statements by police that are contradictory and vague," said Nicholas Bequelin, a researcher for Human Rights Watch who is based in Hong Kong.

"The use of lethal force is extraordinary. There is no justification for shooting dead five people in these conditions."

The security clampdown has filtered across all areas of society. Several dissidents have been detained, petitioners have been sent to labour camps and countless migrant workers have been told to leave Beijing.

A tightening of visas has hit foreign students, NGO workers and businessmen. Passport checks and door-to-door inspections have been stepped up.

Among the most recent victims is Dechan Pemba, a British national of Tibetan ancestry who was deported on Tuesday without being allowed to call the British consulate.

The Chinese foreign ministry said the 30-year-old Mandarin student and part-time teacher was a member of the Tibetan Youth Congress, which China considers a terrorist group.

Pemba said the allegations were absurd. Although she worked for the less radical International Campaign for Tibet in Berlin until 2005, she said she had no contact with any Tibetan groups since arriving in China in 2006.
"It's completely untrue, ridiculous," she told the Guardian, after arriving back in the UK. "I was not involved in any activity that could be considered anti-Chinese or illegal in the two years I was in China."

She blamed her deportation on pre-games paranoia. She had been questioned on two earlier occasions this year. Others with Tibetan backgrounds have suffered similar or worse treatment, she said.

"I am worried about Tibetans who live in Beijing," she said, "If they can just suddenly deport people like me who are living there legally without allowing a call to an embassy, what can they do to any Tibetans who they want to remove?"