Beijing (CNN) -- For the first time, urban dwellers in China now outnumber those living in the country.
At the end of 2011, China counted 690.79 million urban dwellers, the National Bureau of Statistics (NBS) said Tuesday. That marks an increase of 21 million over the previous year and accounts for 51.27% of the country's 1.347 billion people.
During the same period, the rural population shrunk by 14.56 million to 656.56 million.
The number of people between the ages of 15 and 64 stood at about 1 billion, or 74.4% of the total population, the NBS added.
China's breakneck economic growth over the past three decades has seen an explosion in the size of China's cities and towns, especially along its prosperous east coast.
Spurred by Deng Xiaoping's reform policies of the 1970s, which were aimed at modernizing an economy that had been dominated by agriculture, millions of Chinese farmers have been on the move, a massive internal migration that has changed the face of China.
Many of these migrant workers, known as "mingong," have left the countryside for the cities looking for better-paid jobs in areas such as manufacturing.
Mega-cities like Shanghai, Beijing and Guangzhou, have been at the heart of this change, serving as the engines of China's rapid economic transformation.
"Urbanization is an irreversible process and in the next 20 years, China's urban population will reach 75% of the total population," said Li Jianmin, head of the Institute of Population and Development Research at Nankai University, in quotes carried by Agence France-Presse.
"This will have a huge impact on China's environment, and on social and economic development."
There are at least 200 million migrant workers in the whole country, according to a national census carried out in April last year. Many are seasonal workers who work in cities part-time and then return to their villages -- though an increasing number live and work in urban centers permanently.
However, it is difficult for many rural migrants working in cities to become fully-fledged urban residents, due to a centuries-old household registration system known as "hukou," which categorizes the population into rural and urban residents.
Without an urban hukou permit, a migrant is often denied access to the subsidized health, housing and education for children that city dwellers enjoy.