For specialists in China's internet controls, the National People's Congress this weekend is a chance to measure the state of the art of web censorship.

The authorities set the tone last week, summoning the managers of the country’s main internet service providers, major portals and internet cafe chains and warning them against allowing "subversive content" to appear online.

Such actions are in keeping with a trend aimed at assigning greater responsibility to internet providers to assist the government and its army of up to 50,000 internet police.

"If you say something the web administrator doesn't like, they'll simply block your account," said Bill Xia, a US-based expert in Chinese internet censorship, "and if you keep at it, you'll gradually face more and more difficulties and may land in real trouble."

According to Amnesty International, arrests for the dissemination of information or beliefs via the internet have been increasing rapidly in China, snaring students, political dissidents and practitioners of the banned spiritual movement Falun Gong, but also many writers, lawyers, teachers and ordinary workers.

In the last year or so, experts say the country has gone from so-called "dumb internet controls", which involve techniques like the outright blocking of foreign sites and the monitoring of specific e-mail addresses, to far more sophisticated measures.

Newer technologies allow the authorities to search e-mail messages in real time, trawling through the body of a message for sensitive material and instantaneously blocking delivery or pinpointing the offender.

Other technologies sometimes redirect internet searches from popular companies such as Google to copycat sites operated by the government, serving up sanitised search results.

But many experts said the sophistication and sheer number of Chinese users was also increasing rapidly, making it ever more difficult for the censors to prevail. China has 94 million users and usage, most of it broadband, is growing at double-digit rates every year.