The residence registration system, or hukou, is one of the most influential tools of social control in China. Under it, people can legally live and work only where they are registered. Urban residents are entitled to benefits such as unemployment subsidies, pensions, medical insurance and free housing. But rural residents often have nothing except the land given to them under the 1970s household responsibility system.

Mobility was freer in the early days of communist rule. But the influx of peasants into cities troubled the Government - eager to keep a labour surplus in the countryside - and the first legislation on residence control was promulgated in 1958. Migration to cities is not easy. Controls on movement are almost as strict as migration between countries. The wealthier the city, the harder it is to get a residence permit.

Market reform in the past 20 years created a huge floating population - estimated at 100 million. After much chaos and public furore in the early 1990s, the Government issued temporary residence permits. But many migrants stayed without documentation - partly to avoid the red-tape, discrimination and abuse encountered when applying for permits.