Dense smog shrouded Beijing on Saturday, with pollution at hazardous 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 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 warning centre said.

The quality is considered good when the figure stands at less than 100.

But a reading shown on the United States embassy website in Beijing was above 800. Beijing only measures up to a maximum value of 500, with the US embassy tweeting that their own readings were "beyond index".

Last year Beijing said it was illegal for foreign embassies to issue their own air quality readings, but the US said its diplomatic missions in China would not stop tweeting levels, which were useful to its citizens living abroad.

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 also covered vast swathes of east and central China also closed numerous highways and delayed flights in several provinces, it added.

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.