Millions of counters set out to tally the world's biggest population in China, which was estimated last year to be over 1.3 billion.
More than six million census takers are expected to gather the latest data on China's population, including the nation's unprecedented urbanisation drive and the latest results on its controversial "one child" family planning policy.
The government has promised complete confidentiality during the month-long count, although census takers will be able to call for police help if people refuse to take part, the official China Daily newspaper said.
Officials have acknowledged that the count will be a difficult task due in part to the nation's huge migrant population, which is wary of giving away information that could land them in trouble with the law.
An estimated 211 million people make up China's "floating population" -- an army of migrant labourers descending on cities and towns in an unprecedented wave of urbanisation.
Besides making a shambles of China's strict "household registration system" that for decades has registered one person in one particular area, many migrant workers have also violated the "one child" population control policy.
In the 2000 census, China's population was calculated at 1.29 billion, compared to 594 million in the first census in 1953.
The count will also provide a snapshot of China's growing gender imbalance -- often linked to the "one child" policy and a preference for boys -- which has resulted in millions of men of marrying age without wives.
In addition, fresh data on China's quickly ageing population will be gathered, officials have said.
This census also marks the first time China will count the number of foreigners living and working in the country.