Shenzhen, China - It's nightfall in China's most watched city. Women nervously draw tight their curtains, fearful that the cameras that record their daily trips to the office and shops will pry into their homes and secretly film their most intimate moments.

It is no empty fear. Shenzhen, a frenetic city of 12 million on the border with Hong Kong, is at the heart of an experiment by the national government to monitor its vast population with the latest closed-circuit television, or CCTV, technology - and the boundaries of the experiment are being questioned.

More than 200,000 hi-tech cameras disguised as lighting columns have been set up at main streets, shopping plazas, parks and highways in Shenzhen, beaming live video to monitors in what was expected to become the world's most extensive use of technology for social control.

From the moment you walk over the land border from Hong Kong into the bustling city that just 20 years ago was a sleepy fishing village, high-tech cameras with a 
360-degree scope follow you.

Within two years, Shenzhen was expected to have video feeding into a central database from as many as 2 million surveillance cameras - the highest concentration on the planet and four times more than London.

The experiment is part of a nationwide project called Golden Shield, designed to tighten social controls with the use of video technology, much of it imported from major Western companies as security for this month's Beijing Olympic Games.

In Shenzhen, according to deputy police chief Shen Shao Bao, the experiment is already paying golden dividends. Crime rates have fallen by more than 10 per cent and police detection rates have risen by 2.6 per cent since the army of silent sentries began surveillance duty in 2006, he said.

Under the rule of the Communist Party where social control is the norm, the sudden appearance of lamppost-style security cameras across the city was never challenged - until an incident in May stoked concerns that the technology might turn out to be a double-edged sword.

At the Wang Ye Gardens apartment block 3 kilometres from the Hong Kong border, residents noticed that a rooftop surveillance camera on a neighbouring block supposedly monitoring traffic swiveled around from midnight to 5 am every day and trained its high-powered lens through their windows.

A local journalist monitored a government website on which CCTV footage from cameras across the city was made openly available and saw that the camera was scanning the apartment block for lighted windows and filming naked women in their bedrooms and bathrooms. The revelation almost caused a riot.

"People were very, very angry," a spokesman for Wang Ye Gardens management said. "Some of them wanted to go out and smash all the CCTV cameras. They are still very unhappy about the cameras - but they are government property, and we can't move them."

Instead, it served as a wake-up call to the extent to which public surveillance has developed in Shenzhen.

"It is like the Big Brother era in 1984," said one of the estate's residents, Li Xiang. "After this incident, I realized that as soon as you step out of your door in this city, you are under CCTV surveillance."

It is not only the residents of Wang Ye Gardens who are concerned about the pervasive use of CCTV cameras. In a one-party state where activists are jailed for years for opposing the Communist Party, human rights groups said they fear the technology would be used to identify and arrest dissidents.

"Activists aren't only followed by secret police in Shenzhen anymore," said one Hong Kong-based activist who meets regularly with underground labour rights groups in China. "They are concerned that they are followed everywhere by CCTV cameras as well.

"They may not have face-recognition technology on these CCTV cameras yet," the activist said, "but they can easily be upgraded when the technology is available, and no one will know."

But according to Xue Jun Ling, a project manager with Shenzhen Xinhuo Electronic Engineering, which set up a bank of 38 surveillance cameras around the city's Civic Centre, person-recognition technology is already being widely used.

That technology allows the irises, facial features or even the walking mannerisms of subjects to be checked electronically against a database of people the police or government security services want to monitor.

"We already use face-recognition technology in government offices and in the entrance to shopping malls in Shenzhen," Xue said.

"The technology is being upgraded day by day, and the connectivity of different parts of the system to a central monitoring desk is also being constantly improved," he said. "It is a nationwide movement in China. Even small towns are trying to build up their own systems even though they don't have the resources that we have here."

In an extraordinarily frank admission, Xue added: "We did mobile surveillance during the Olympics for the Ministry of State Security. We have surveillance equipment inside a plain vehicle. We park it at Olympics venues, and it sends data and pictures by satellites to the ministry."

A China-based human rights activist who asked not to be named for her own safety said: "We're worried that the government is using the Olympics as an excuse to import this technology from Western companies. Now the games are over, we are worried they will use that technology to identify and round up dissidents."

Xue saw no such dark clouds on the horizon. "We are the national leaders in this type of technology," he said proudly. "We are the city of peace, harmony and security."

For the residents of Wang Ye Gardens, Shenzhen is also the city of unwarranted intrusion. Embarrassed by last month's revelations, the city government blocked public access to the government website showing CCTV footage.

"It isn't good enough," professor He Bin of Beijing's China University of Political Science and Law complained to a Shenzhen newspaper. "Why didn't the government disclose the names of the people responsible and reveal the full details of the case?"

As far as human rights groups are concerned, CCTV monitoring could manifest itself in far more sinister and threatening ways than grainy footage of naked women in the windows of apartment blocks. (dpa)