Mask-wearing Beijing residents endured a third day of pollution at hazardous levels on Sunday, as authorities warned that a thick cloud of smog may not lift from the Chinese capital until midweek.

As the dense smog shrouded large swathes of northern China, flights were cancelled and traffic delayed as visibility was reduced to 100 metres in some areas.

The dangerously poor air quality was highlighted by a steady stream of news broadcasts on state television, many of which warned residents against venturing outside.

The pollution also provoked China's huge number of microbloggers to take to the internet, with some high-profile web-users calling for a re-evaluation of China's rampant modernisation.

Rapid economic growth has led to a dramatic increase in the consumption of coal and car-clogged city streets.

"The foreign media is laughing at us. I agree with their laughter," said Hu Xijin, the editor of the state-run Global Times newspaper on Weibo, China's version of Twitter.

"This is a warning to the Government and Beijing's citizens. We have to think about what kind of modernisation we want and how to manage it."

Hu said the pollution issue would be on the front page of his newspaper on Monday.

Beijing's municipal environment warning centre issued its second alert in two days, warning people to avoid outdoor physical activity.

The centre also urged government officials to set an example to other residents, by not using their cars.

Doctors at the Beijing Chaoyang Hospital and the Beijing Children's Hospital said that the number of patients with respiratory disease had jumped sharply in the past few days, according to Xinhua news agency.

It also reported that the municipal authorities have ordered outdoor sports to be stopped at primary and middle schools in the most polluted areas until Tuesday.

Production halted at the Beijing Hyundai Motor Company on Sunday and construction has been suspended at 28 building sites, according to the city's Municipal Environmental Protection Bureau, Xinhua said.

Air quality in Beijing showed small airborne particles with a diameter small enough to deeply penetrate the lungs at a reading as high as 993 micrograms per cubic metre on Saturday evening, the warning centre said.

The World Health Organization says the figure for such particles, known as PM2.5, should ideally be no more than 25 micrograms per cubic metre.

High levels have been linked to health problems including respiratory disease, heart disease and lung cancer.

A reading shown on the United States embassy website in Beijing was above 800 micrograms per cubic metre at the height of the pollution Saturday. But it was at 375 at 5pm on Sunday (0900 GMT).

Official PM2.5 figures have only been monitored in China's major cities since the beginning of last year.

The lack of official data makes it difficult to compare the recent smog with previous cases of pollution, said Ma Jun, director of the Beijing-based Institute of Public and Environmental Affairs.

"But it is interesting to see that residents have been warned about the dangers of pollution by official media," Ma told AFP.

"This has never happened before, and is a result of the transparency that we now have with the figures."

The pollution is expected to engulf Beijing until Wednesday before wind arrives to blow it away, according to a weather report from the meteorological station, Xinhua said.

Ten flights had been cancelled and five delayed by more than an hour at Beijing Airport by 11am (0300 GMT) Sunday, the airport said. However, flights were departing on schedule by the afternoon.

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.