ense smog has shrouded Beijing, China's capital, with pollution at hazardous levels for a second day, with residents advised to stay indoors, state media has said.

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

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 website of the US embassy in the city was above 800.