The tops of some skyscrapers were obscured by the sandstorm - caused by winds from drought-hit Mongolia and northwestern China - with pedestrians forced to cover their eyes as gusts of dust swept through the streets.

"It's quite serious today. There's always a day or two like this (of pollution or dust) each month," said Mr. Fan, 39, who did not wish to disclose his full name.

Beijing's official air quality index reached a maximum level of 500 on Sunday morning, with floating particles known as PM10 surpassing 2000 micrograms per cubic metre in some districts.

The World Health Organization recommends average daily PM10 concentrations of no more than 50 micrograms.

The China Meteorological Administration issued a yellow alert on Friday, warning a sandstorm was spreading from Mongolia into northern Chinese provinces including Inner Mongolia, Shanxi, Liaoning and Hebei, which surrounds Beijing.

Beijing faces regular sandstorms in March and April due to its proximity to the massive Gobi desert as well as deforestation and soil erosion throughout northern China.

China has been trying to reforest and restore the ecology of the region in order to limit how much sand is blown into the capital.

Beijing has planted a 'great green wall' of trees to trap incoming dust, and has also tried to create air corridors that channel the wind and allow sand and other pollutants to pass through more quickly.

Australian Associated Press