A Chinese police chief has said he uses more than 12,000 spies to inform on a remote county of just 400,000 people, an admission that lays bare the enormous scale of China's surveillance network.

Liu Xingchen, the 56-year-old assistant to the head of Kailu County, a farming region in Inner Mongolia, said his vast network of informants meant he could be "very sensitive" to any signs of dissent and protest.

In an interview with Xinhua, the government-run news agency, Mr Liu described how he was able to "quickly and accurately discover all sorts of information that might destabilise society".

"Every policeman and auxiliary policeman, no matter their division or particular police station, has to establish at least 20 informants in their community, village, work unit and so on. Altogether, these add up to 10,000 spies.

"Then the actual criminal units, the economic crimes unit, the Domestic Security Department, the Public Information Security Supervision and so on will establish a further five 'eyes and ears'.

"At the latest count, our bureau has established 12,093 informants," he said.

Mr Liu also said that the bureau had "evolved" from being passive guardians of the law to active ones. "We have gone from punishing people after the crime to resolving the problem before it happens," he said.

Experts said the number of spies in China's major cities, such as Beijing and Shanghai, and in more restive regions, such as Tibet and Xinjiang, was likely to be far higher. The number of spies in Kailu County, extrapolated nationwide, suggests China has at least 39 million informants, around three per cent of its population. By comparison, around 2.5 per cent of East Germans spied for the Stasi secret police under Communism.

It is unclear whether all the informants in Kailu County were kept on the government payroll, but other Chinese cities have adopted a rewards system. More than 200,000 yuan (£18,730) was awarded in a single month in the southern city of Shenzhen to informants who offered 2,000 tips on criminal activity.

Meanwhile, researchers at China Digital Times have translated leaked internal documents that spell out the role of China's Domestic Security Department (DSD), the huge security operation that is dedicated to "preserving public harmony".

The DSD keeps watch over anyone with "distinct views in the economic, cultural and political domain" who "possess different views from the authorities and insist on expressing them".

"We should persist in putting punishment first; strike and take care of things early," the documents state.