Running late for her English-teaching job at a private school, Ms Pemba stepped out of her flat and was surrounded by seven to eight security agents. Within 15 hours she found herself back in London - and banned from returning to China for five years.
The security agents detained Ms Pemba for an hour in her flat. They confiscated her passport, phones and the prized Olympic tickets she'd secured in the national lottery open to bona fide residents of China.
They refused to let her contact anyone, even the school. They took her Chinese bank book and demanded her PIN code before allowing her to throw some belongings into a bag. She was taken to the airport in a convoy, whisked through VIP security channels, and put on a plane.
Ms Pemba was given her mobile phone and passport back as the plane was about to leave. She had just minutes to call a friend and the British embassy, 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."
It turned out Ms Pemba's "crime" was being ethnically Tibetan, and she had previously been active with the International Campaign for Tibet, a Tibet independence group that supports the Dalai Lama. She is also the niece of a prominent Tibetan scholar and China critic, Tsering Shakya, who teaches in Canada.
But these were all long known to the Chinese Government which had repeatedly issued her with work visas, allowed her to sign a lease and to pay tax.
Ms Pemba says since she moved to Beijing to study Mandarin she had stayed away from activism, and had done nothing that could have been construed as "anti-China", let alone illegal.
While the Herald cannot independently verify Ms Pemba's account, which she says was videotaped by two officers, it is in line with a widespread security crackdown that is netting and expelling thousands of "undesirables", Chinese and foreign, before the Olympics.
Beggars, the homeless, prostitutes, scrap recyclers, bottle collectors, activists and anyone who might challenge Beijing's efforts to present itself as a modern, sophisticated metropolis are being forced to leave or cease work. A tightening of visa regulations this year has also seen thousands of long-term residents who had been relying on annual business visas suddenly refused entry.
Security officials believe the biggest threats to the Games comes from Tibetan independence forces, Muslim separatists from the western province of Xinjiang and established critics of the regime such as members of the Falun Gong spiritual movement.
This week state media reported that police shot dead five Muslim Uygurs in Urumqi, Xinjiang's capital, who they alleged had wanted to start a "holy war".
The dissident writer Liu Xiaobo, who remains under police surveillance but has not been arrested, said the Government was in a state of extreme nervousness, or "caomu jiebing", an idiom referring to heightened fear where even the grasses and trees look like enemies. Security experts and others close to Olympic Games planning say that even with China's vast experience in suppressing dissent and maintaining order, the Government will be severely challenged by the influx of 2.5 million visitors, including 550,000 foreigners and more than 30,000 media representatives. They say the authorities are "totally on edge", verging on paranoia, about the possibility of disruptions that would embarrass China.
Long-term foreign residents have complained that it seemed the authorities did not care if they turned Beijing into a sterile fortress if it prevented protests. One foreign Olympic source disagreed. "It's not that they don't care, they just don't understand," the source said.
The authoritarian Chinese system was not used to having to take into consideration issues such as public convenience and the expectations of large numbers of foreign visitors and especially media who would chafe under restrictions that locals are accustomed to, the source said.
Foreign broadcasters including the BBC are still fighting for the right to broadcast continuously from Tiananmen Square and a German station that had permission to film from the Great Wall has repeatedly had its coverage interrupted by police.
But a far bigger problem, the source said, was the pervasive secrecy that permeated the Government, military and police bureaucracy, making security forces reluctant to communicate with each other, let alone with other key parties. This had created situations where even the Beijing Olympic Organising Committee, venue managers and athletes were not being briefed to the extent necessary.
"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 … here there's no integration," the source said.
The expert also said the well-publicised terrorism drills and biological and chemical threat exercises were mostly public relations stunts. "They are paranoid, even the military doesn't have the experience or the training or equipment to do this properly," the source said, giving as an example a SWAT commander in a remote province who said without embarrassment that he based his training methods on watching US TV shows because there was no money or resources for professional training.
Another Western security expert said that for all Beijing's 300,000 security cameras, the key was how well plans were executed. "It will be an interesting couple of weeks," the expert said.
This source said he believed Chinese police were rightfully "fully focused" rather than paranoid, even though 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 rather than Beijing," the expert said.
He said Xinjiang extremists weren't amateurs but "they're not the same threat as al-Qaeda is in other countries". At the same time, he and other foreign experts admitted it was difficult to assess the veracity of internal threats in China because the Chinese did not share this information.
Liu said despite the extensive precautions being taken to prevent negative incidents occurring in Beijing, "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".
But he said that because of the nature of this simmering discontent, such acts would be difficult to prevent because they were more likely to be ad hoc, unplanned and with no clear political aims. He cited this month's attack on a Shanghai police station in which six officers were stabbed to death by a man angry at being accused of stealing a bicycle.
An Australian security expert, Neil Fergus, an adviser to the Games after working on the Sydney, Salt Lake City and Athens Games, said it was a "tightrope" to balance security imperatives while still enabling the athletes, visitors and locals to enjoy the sporting event.
Asked if Beijing was getting the balance right between strict security and a relaxed atmosphere, Alexandra Oikonomidou, the director of Olympic and sports marketing for the public relations giant Ogilvy said the two were related.
"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 more relaxed and secure inside the Olympic venues," she said.