The photo was one of a series taken by local media covering a police raid on prostitution and gambling dens in Zhengzhou, capital of Henan province, said the official China Daily, which also published the controversial picture.

"Police in the mainland grab a person's hair in the process of law enforcement, and forcefully take naked photos by violent means," one online user said yesterday on popular web portal Chinaren.

"This is a criminal offence - much more serious than prostitution!"

A video clip of the raid, also widely available online, shows the plainclothes policeman bash down a door in what appears to be an upscale massage parlour.

"How many have you had tonight?" the policeman says to a naked woman sitting down on the floor - an alleged prostitute - as a client, also naked, crouches near her.

The hair-pulling incident only appears in the still photo, not the video.

A spokeswoman at the Zhengzhou police bureau, who refused to be named, told AFP an investigation into the case was ongoing, but refused to provide any further details.

China has the world's largest online population of at least 338 million users, and as traditional media remains tightly controlled, the Internet has become a platform for ordinary people to vent their frustrations.

In one well-known example, a 21-year-old waitress in central China walked free from a trial in June despite stabbing to death an official who demanded sex after the case sparked nationwide Internet outrage about government sleaze.

A poll published on a news website affiliated with China's Supreme People's Procuratorate revealed that 57 percent opposed the publication of the latest picture because "prostitutes also have basic human rights."

Another 35 percent also opposed the method but blamed the media, not the police, for publicising the pictures, according to the survey, which had attracted nearly 870 respondents yesterday.