The policewoman said Shuangjing police station issues a large number of police registration cards. The area is popular with foreigners for two reasons: It is in close proximity to the central business district and also the apartments are relatively modern. "I hear that the number of foreigners registered with Nanhu police station near the Wangjing area is the highest in Beijing and the majority of them come from the Republic of Korea," she added, explaining that locals often refer to the area as "Korean Town".

Citizens of the ROK, the US, Canada, Russia and Japan were the top five nationalities involved in "three illegal" cases in 2011, according to the exit-entry administration of the Beijing Municipal Public Security Bureau.

More than 20,000 "three illegal" foreigners were dealt with nationwide last year, according to Yang Huanning, vice-minister of public security.

However, the crackdown has provoked controversy within Beijing's expatriate community and the overwhelming reaction has been one of concern.

Ismael de Pierrepont, a 29-year-old Frenchman who works for an online Chinese retail company, said some friends were recently asked by police to show their passports in the Sanlitun area of Beijing, a popular spot for foreigners. "One of my friends had to sneak out of a bar through the back door, because he didn't have his passport with him," he said. "Since then I have carried my passport with me at all times."

Having lived in Beijing for four years, De Pierrepont said he wasn't surprised by the campaign. "I've had the police knock on my door and ask to see my passport, without giving a reason. It has happened twice in the past four years," he said. "I didn't say anything. I simply showed them my passport.

"I speak Chinese, I have Chinese friends and I use Sina Weibo (China's leading micro blog service). I know what's going on in China and I know the rules," he said. "If the government asks you to do something, you do it."

What concerns him is not the policy itself, but the attitude that surrounds it. "In China, I always feel so different," he said. "People treat you differently, for good or bad, just because you are foreign."

Take the phrase laowai, the common Chinese term for foreigners: De Pierrepont said it can sometimes be used as a pejorative term rather than simply a neutral description. "The word sometimes sets the two groups, Chinese and foreigners, in opposition," he said.

Age-old enmity can also play a role, something De Pierrepont discovered when visiting Yuanmingyuan (Old Summer Palace) in Beijing, a couple of years ago. After discovering his nationality, a Chinese visitor told De Pierrepont, "I don't like French people." His prejudice stemmed from the days of the Second Opium War (1856-1860), when French expeditionary forces looted and destroyed the former royal retreat.

"I didn't say anything. I didn't know what to say," he said. "China is not a nation of immigrants. People don't know how to deal with people from different countries and cultures. I hope the situation improves as quickly as China is developing."

"His experience was very rare in China. Most Chinese treat foreigners in a very friendly way, sometimes too friendly," said Qiu Lin, a senior associate at a public relations company.