China said Tuesday the number of people living in cities exceeded the rural population for the first time, a historic shift that experts said would put a strain on society and the environment.

The change marks a turning point for China, which for centuries was a mainly agrarian nation but has witnessed a huge population shift to cities over the past three decades as people seek to benefit from rapid economic growth.

Urban dwellers now represent 51.27 percent of China's entire population of nearly 1.35 billion -- or 690.8 million people -- the National Bureau of Statistics (NBS) said.

It added that China had an extra 21 million people living in cities by the end of 2011 compared to a year earlier -- more than the entire population of Sri Lanka -- while the number of rural dwellers dropped.

"Urbanisation is an irreversible process and in the next 20 years, China's urban population will reach 75 percent of the total population," said Li Jianmin, head of the Institute of Population and Development Research at Nankai University.

"This will have a huge impact on China's environment, and on social and economic development."

A significant portion of those moving to cities are migrant workers -- rural residents seeking work in urban areas -- who have helped fuel growth in the world's second-largest economy.

A national census published in April last year showed China counted more than 221 million migrants, and a government report released months later predicted that more than 100 million farmers would move to cities by 2020.

But migrants are often treated as second-class citizens in the towns or cities they live in, as they are still registered as rural residents and have little or no social security.

"We're already seeing some of the destabilising aspects of (urbanisation) because China's political and administrative system hasn't caught up with the economic and social reality," said Geoffrey Crothall, spokesman for the Hong Kong-based China Labour Bulletin.

He said many people still classified as rural residents now formed major portions of the population in many cities around China.

As such, many migrant workers -- especially the younger, so-called second generation -- are increasingly frustrated with the treatment they receive, and the issue has in some cases sparked violent unrest.

In June last year, three days of riots broke out in the southern province of Guangdong after rumours spread that police had beaten a street hawker to death and manhandled his pregnant wife.

Li said land grabs -- an issue that has fuelled countless riots and violent protests around China -- were also part of the problem.

"A lot of farmers are actually forced to become urban dwellers when their land is expropriated, but they aren't able to find work in cities," he said.

"China is still not clear about the meaning of civil and property rights, and there are no appropriate measures to protect them, which will inevitably lead to corruption and social injustice," he said.

According to official figures, the rate of urbanisation in China has gathered pace over the past decades.

In 1982, only one in five people lived in cities. By 1990, urban dwellers represented 26 percent of the total population, a figure that rose to 36 percent in 2000 and jumped faster over the next decade to reach 51.27 percent.

Li said the rising number of urban dwellers would put a strain on resources as new or expanded cities would have to be built, with more transport, energy, water plants and other infrastructure needed to cater for more people.

He added that different cities had adopted different attitudes towards the issue.

"Big cities such as Beijing and Shanghai have already clearly stated they want to contain the population increase," he said.

They "have implemented a number of measures that are necessary as it is a severe test for local resources and traffic."

But he said some small and medium-sized cities were still actively encouraging the rural population to become urbanites, which put a strain on resources and could pollute the local environment.