BEIJING: Dense fog enveloped swathes of east and central China Saturday, with pollution levels in Beijing reaching dangerous levels for a second day and residents advised to stay indoors, state media said.

The municipal environment warning centre issued an alert advising the elderly, children, and those suffering respiratory or cardiovascular illness in the capital to avoid going out or doing strenuous exercise, Xinhua reported.

Those who did venture out wore facemasks for protection, with visibility low, the skyline shrouded, and the sun hidden in the smog.

Air quality in Beijing showed airborne particles with a diameter small enough to deeply penetrate the lungs at a reading of 456 micrograms per cubic metre. The quality is considered good when the figure stands at less than 100.

The heavy pollution is expected to last another three days, with weather conditions preventing pollutants from dispersing, the warning centre said, according to Xinhua.

Fog in several provinces in east and central China closed numerous highways and delayed flights, it said.

China's air quality is among the worst in the world, international organisations say, citing massive coal consumption and car-choked city streets in the world's biggest auto market.