Media playback is unsupported on your device Media caption One Beijing resident explains she was forced to stay indoors because of the smog

Residents of China's capital, Beijing, have been warned to take precautions after air pollution readings soared.

Readings on Thursday registered more than 20 times the recommended exposure levels by the World Health Organisation (WHO).

Thick smog reduced visibility to a few hundred metres, shrouding skyscrapers and leaving an acrid smell in the air.

Officials advised people going to work to wear protective masks, and children and the elderly to stay indoors.

WHO guidelines say average concentrations of the tiniest pollution particles - called PM2.5 - should be no more than 25 microgrammes per cubic metre.

On Thursday, official readings for PM2.5 at one point showed more than 500 microgrammes per cubic metre.

Another monitoring post at the US embassy said the pollution level was briefly more than 25 times higher than the amount considered safe by WHO.

Four highways linking Beijing to other cities were temporarily closed on Thursday morning due to poor visibility, Xinhua news agency says.

Image copyright AP Image caption Beijing is often hit by thick smog caused by air pollution

Image copyright AFP Image caption People were advised to wear protective masks while outdoors

Image copyright AP Image caption There were reports that there was an acrid smell in the air

"I couldn't see the tall buildings across the street this morning,'' one man named Zhang, a traffic co-ordinator, told the Associated Press news agency.

China's cities are frequently blanketed by pollution caused by coal-burning power plants, factories, and millions of vehicles on the roads, says the BBC's Martin Patience in Beijing.

Studies estimate that air pollution causes hundreds of thousands of premature deaths every year, our correspondent adds.

Following widespread criticism from the public and the state-run media, officials pledged to do more to clean up the air after pollution levels in Beijing reached hazardous levels last year.