Ms Pemba, a Tibetan by ethnicity, stepped out of her Beijing flat on Tuesday morning and headed for her English-teaching job at a private school. She was surrounded by about eight security agents and within 15 hours was back in London, banned from returning to China for five years.

The security agents detained her for an hour in her flat, confiscated her passport, bank book, mobile phone and the prized Olympic tickets she had won in a lottery open to residents. They refused to let her contact anyone.

She was allowed to throw some belongings in a bag before being taken to the airport in a convoy and put on a plane. Her phone and passport were returned as the plane was about to leave, giving her just minutes to call a friend and British embassy staff, who informed her parents in London.

Asked why she was being deported, the security agents told her: "You ought to know what you've done wrong".

Ms Pemba's "crime" is being ethnic Tibetan and previously active with the International Campaign for Tibet, which supports the Dalai Lama. She is also the niece of prominent Tibetan scholar and China critic Tsering Shakya, who teaches in Canada all facts long known to the Chinese Government, which has issued her repeat work visas, allowed her to sign a lease and pay Chinese tax.

Ms Pemba says that since she moved to Beijing two years ago to study Mandarin, she has stayed away from activism and insists she has done nothing that could be construed as "anti-China", let alone illegal.

While The Age cannot independently verify Ms Pemba's account of her deportation, which she says was videotaped by two officers, it is in line with a widespread security crackdown that is expelling thousands of "undesirables", both Chinese and foreign, ahead of the Games.

Beggars, the homeless, prostitutes, activists, and anyone who might challenge Beijing's efforts to present itself as a modern, sophisticated metropolis are being forced to leave, or denied re-entry.

Security officials believe the biggest threat to the Games is from Tibetan independence activists and separatists from the western province of Xinjiang, which has a big Muslim population. They are also worried about members of the Falun Gong spiritual movement, banned in China as an "evil cult". Its members are regularly persecuted and tortured.

This week, state media reported that police shot dead five Muslims in Urumqi, Xinjiang's capital, who allegedly wanted to start a "holy war". Officers opened fire after a gang of five women and 10 men rushed at them with knives, shouting about "perishing together" and "sacrifice for Allah".

Police say they have detained 82 suspected terrorists this year in Xinjiang for planning to attack the Olympics.

Security experts and others close to Games planning say China's vast experience in suppressing dissent and maintaining order will be severely challenged by 2.5 million Olympic visitors, including 550,000 foreigners and more than 30,000 journalists.

Sources familiar with the security planning say authorities are "totally on edge", verging on paranoid, about possible disruptions that would embarrass China.

One foreign Olympic source says the pervasive secrecy that permeates China's bureaucracy is a significant problem, making it difficult for security agencies to share information.

The source said "the left hand doesn't know what the right hand is doing it's very fragmented and the most important element (of Olympic security) is that integration here there's no integration. Security? We don't talk about that."

The well-publicised terrorism drills and biological exercises were mostly public relations stunts. "They are paranoid. Even the military doesn't have experience or the training or equipment to do this properly I know of one SWAT commander in a remote province who said that he got his training methods from watching US television shows.

"They just don't have the money for new facilities and the kind of training they need."

Another Western security expert said he believed Chinese police were fully focused rather than paranoid, and he believed the threat in Beijing, which has a tightly controlled environment, was lower than at the Athens Games. "If you were a terrorist and planning an event, you'd aim towards London (in 2012) rather than Beijing," he said.

While Xinjiang extremists weren't amateurs, he said, "they're not the same threat as al-Qaeda is in other countries".

Dissident writer Liu Xiaobo, who remains under police surveillance but has not been arrested so far, said the Government was extremely nervous.

Mr Liu said that despite extensive precautions, "long-time anger and discontentment is so great that such conflict from within China may take the chance of the Olympics and create a bigger threat than terrorists from outside the country". Mr Liu said such acts would be difficult to prevent because they were often unplanned and with no clear political aim. He cited this month's attack at a Shanghai police station in which six officers were stabbed to death by a man angry at being accused of stealing a bicycle.

Australian security expert Neil Fergus, who works as a consultant to the Beijing Games, after working on the Sydney, Salt Lake City and Athens games, believes China must regard al-Qaeda as a credible threat via a home-grown group: the East Turkistan Islamic Movement, which has declared allegiance to al-Qaeda and has been responsible for various bombings over the past decade.

Mr Fergus has said that it was a "tightrope" balancing security needs while still allowing athletes, visitors and locals to enjoy the Games.

Alexandra Oikonomidou, director of Olympic and sports marketing practice for public relations giant Ogilvy PR Worldwide, which is advising various Olympic sponsors, said that given how serious global terrorism had become and the attention the Beijing Games was attracting, "it clearly makes sense for China to be so well-prepared".

Asked if Beijing was getting the balance right between strict security measures and public convenience and a relaxed atmosphere, she said the two were related.

"Without high security measures we cannot enjoy convenience and a relaxed and free-flowing atmosphere for all the Olympic athletes and visitors to the Games. The Olympic family and fans are used to the Olympic security regulations," she said.

"For those who are going to attend the Games for the first time, it will be slightly inconvenient at the beginning, but these regulations will make them feel relaxed and secure inside the Olympic venues."