Shanghai has been conspicuously absent from a roster of Chinese cities engaged in a high-profile nationwide campaign against prostitution, giving rise to speculation local police want to keep a low profile for visitors during the World Expo.

Since April, 26 cities have begun concerted drives against prostitution - along with gambling and drug abuse - which have, so far, enjoyed extensive media coverage.

Indeed, police from many major cities have scored so many successes that they issued monthly progress reports. Shanghai, by contrast, has made very few of its crackdowns known.

But Zhuang Liqiang, a spokesman for the municipal public security bureau, said on Monday that the lack of publicity in Shanghai does not mean local police are shying from the fight. In fact, he said, prostitution-related arrests have become a steady priority for city police.

Zhu Xiaoding, a Beijing-based attorney who has been following the campaign said Shanghai authorities want to downplay the arrests to appear more tolerant of these vices for occasionally indulgent overseas visitors.

"Prostitution is legal in many countries outside of China", said Zhu "Over-emphasis upon an anti-prostitution campaign might hurt the image of Shanghai as an international city in the eyes of foreign visitors."

Anti-prostitution campaigns elsewhere have been topping media headlines since May when police in Beijing took the lead in cracking down on four high-end nightclubs - including the famous Passion Nightclub.

This was followed by a series of similar crackdowns over the following two months. By July 20, Beijing police had broken up some 240 prostitution rings - and detained more than 150 people who trafficked, forced, lured or otherwise introduced women into prostitution.

Successes in Beijing have also been followed by concerted action by police in other cities. Officials in Dongguan, Guangdong province, for example, nabbed almost 1,200 suspects.

Qiao Xinsheng, a law professor from Zhongnan University of Economics and Law, was quoted by the Oriental Morning Post as saying that the campaign is steeped in the government's determination to root out corruption among its own officials - many of whom are believed to be supporting or patronizing venues in the business of prostitution.

This initiative "could not only cleanse social morale", said Qiao, "but may also provide clues to the backdoor corruption of officials."

Zhu, however, noted that local authorities should reconsider their approach to the campaign, given the extreme behavior with which police have made in some of the arrests - in particular by parading prostitutes in public, or by entrapping prostitutes by posing as potential clients.

Such activities, said Zhu, are "clearly violations of human rights".