They allowed her to throw some clothes in a rucksack, then took her in a whizzing convoy to the airport, ordering her deportation on a flight to London.
The expulsion of Pemba, who's of Tibetan descent, is part of a broader campaign to sweep away anyone deemed a potential troublemaker before the Aug. 8-24 Olympic Games. Under the slogan "Peaceful Beijing," security forces wield what human rights monitors call an unprecedented mandate to rid the city of anyone they deem undesirable.
Employing upward of 200,000 closed-circuit television cameras and reviving the use of neighborhood watch committees, authorities have their sights on foreigners with lapsed visas, property activists, petitioners, beggars, prostitutes, street vendors or anyone else who might give Beijing an unsanitary appearance or dare to raise a voice of protest as the world tunes into the Olympics.
Pemba, who's 30, didn't fall into any of those categories. She held a work visa that was valid until November, and she earned a living teaching language part time at a private school. She was perplexed at her deportation.
"When I asked exactly why I was being made to leave, I was told that I ought to know what I had done wrong," she said, adding that the agents told her, "I was forbidden to come back to China for five years."
Pemba was forced to abandon her apartment with most of her belongings still inside.
"My tickets for Olympics events were also confiscated, along with my Bank of China bankbook," she said. "I was also made to give them my PIN number for that bank account."
Pemba was once active in Berlin on behalf of the International Campaign for Tibet, which works closely with the Dalai Lama, the exiled spiritual leader, who lives in India. She said she came to China in 2006 to study Mandarin and had stayed away from activism there.
Official suspicions appear to have been aroused either because of her Tibetan heritage or because she's the niece of Tsering Shakya, a prominent Tibetan historian who teaches at the University of British Columbia in Vancouver. Shakya is a critic of China's iron-fisted control of Tibet.
"I think the Chinese government thinks that any Tibetan is a potential troublemaker," Shakya said in an e-mail. "The Olympics are only a few weeks away and they are paranoid that something will go wrong. All Tibetans who are in the city privately have been told to leave Beijing during the Olympics."
Riots broke out March 14 in Lhasa, Tibet's capital, triggering unrest in the Tibetan areas of China that turned into the worst ethnic rioting in nearly two decades in the country. Angered by China's handling of the protests, pro-Tibet activists disrupted the global Olympic torch relay in Europe in April, embarrassing Beijing and inflaming public sentiment.
A British Embassy spokesman, Bret Sutcliffe, said no other Britons were known to have been expelled from China this year.
Nicholas Bequelin, a researcher based in Hong Kong for Human Rights Watch, an advocacy group, said the magnitude of the sweep was unprecedented, larger than for a 1995 U.N. global conference on women. He said police had mobilized some 100,000 members of neighborhood watch committees as informers, "reviving tactics as they were under Mao," a reference to Mao Zedong, the founder of modern China.
Authorities are worried about any disruption to the Olympics, Bequelin said.
"The slightest attempt to protest, pass a message, unfurl a banner, wear a T-shirt, any kind of expression of support for a cause, is what they want to avoid," Bequelin said.
He said he was concerned that an empowered security apparatus that vigorously used house arrest against dissidents, employed neighborhood snoops and expanded the use of close-circuit TV cameras wouldn't fade away after the Olympics.
"This is going to be the new 'normal' after the games," he said.
Pemba is still getting over jet lag in London after her deportation Monday, and rethinking how 30 or so security agents in four vehicles took part in the operation to get her to the airport and put her aboard an Air China flight.
As she boarded the plane, Pemba said, "They said they hoped I wouldn't cause trouble again."
The officials didn't explain what they'd do with the money in her bank account or with her prized tickets to the Olympics.
"I had about six tickets," she said, including to rowing and track and field events, which she'd waited months to see.