BEIJING (AFP) - Beijing urged residents to stay indoors on Wednesday as emergency measures were rolled out aimed at countering a heavy cloud of smog blanketing the Chinese capital and swathes of the country.

The municipal government said children, the elderly and people sensitive to poor air quality should remain indoors, after authorities announced the closure of 103 factories and ordered 30 per cent of official cars off the road on Tuesday.

As the thick cloud of toxic air choked Beijing for the third consecutive day, the now familiar sight of mask-wearing pedestrians venturing out on heavily-polluted roads was broadcast regularly on state media.

Visibility in central Beijing was reduced to 300m, according to China Central Television, causing 29 flights to be cancelled and another seven delayed.

The state-run broadcaster also showed images of the eastern province of Jiangsu - some 1,000km south of Beijing - covered in a thick blanket of smog.

The United States embassy's air quality index reading for Beijing stood at 336 and "hazardous" at 1pm on Wednesday. The index rates a reading over 150 as "unhealthy" and above 300 as "hazardous".

Meanwhile, the Beijing Municipal Environmental Monitoring Centre gave the figure as 292 at midday, indicating the capital's air was "heavily polluted".

The toxic air is the fourth serious bout in recent weeks, and the winter of smog has sparked an Internet outcry and anger from state media.

China's pollution problems are blamed on the country's rapid urbanisation and dramatic economic development.