A joint immigration and police swoop on mainlanders involved in buying goods in Hong Kong for sale across the border resulted in 20 arrests this week. The operation, codenamed Windsand, ran from Tuesday to Thursday and found people in the city as visitors who were working illegally as so-called parallel goods traders. They were buying goods including milk powder, food, skincare products, electronics and daily necessities with the intention of selling them across the border, dodging higher mainland taxes. Parallel trading in most goods is not illegal in Hong Kong, so any action against the traders is taken under the immigration laws. Seven men and 13 women aged between 27 and 56 were arrested in areas near the border, including Lok Ma Chau and Sheung Shui. None have yet been charged. "The Immigration Department will continue to take enforcement action with related law enforcement departments against the offences concerned," a spokesman for the department said. Since September 2012, some 1,650 mainlanders and 14 Hong Kong residents have been arrested for suspected involvement in parallel goods trading. Of those, 200 were later jailed for breaching conditions of stay and 1,440 were sent back to the mainland, the spokesman said. Parallel goods trading has grown dramatically in recent years and become a key bone of contention between Hongkongers - especially those living in the northern New Territories - and mainlanders. Traders have been accused of clogging up the MTR network, hogging daily necessities and fundamentally altering the retail landscape of large parts of Hong Kong, with traditional stores priced out by those catering to the needs of the traders. Protests have taken place in Sheung Shui against their presence. According to Shenzhen customs, more than 20,000 parallel traders cross the border between Shenzhen and Hong Kong every day, taking advantage of tax regimes which make many goods cheaper in Hong Kong.