SHANGHAI, China -- Shanghai authorities ordered schoolchildren indoors and halted all construction Friday as China's financial hub suffered one its worst bouts of air pollution, bringing visibility down to a few dozen meters and obscuring the city's spectacular skyline.

The financial district was shrouded in a yellow haze and noticeably fewer people walked the city's streets. Vehicle traffic also was thinner, as authorities pulled 30 per cent of government vehicles from the roads. They also banned fireworks and stopped all public sporting events.

Protective masks and air purifiers were selling briskly at local stores.

"I feel like I'm living in clouds of smog," said Zheng Qiaoyun, a local resident who kept her six-month-old son at home. "I have a headache, I'm coughing, and it's hard to breathe on my way to my office."

Shanghai's concentration of tiny, harmful PM 2.5 particles was 602.5 micrograms per cubic meter Friday afternoon, an extremely hazardous level that was the highest since the city began recording such data last December. That compares with the World Health Organization's safety guideline of 25 micrograms.

The dirty air that has gripped Shanghai and its neighbouring provinces for days is attributed to coal burning, car exhaust, factory pollution and weather patterns, and is a stark reminder that pollution is a serious challenge in China.

Beijing, the capital, has seen extremely high smog several times over the past year. In the far northeastern city of Harbin, some monitoring sites reported PM 2.5 rates up to 1,000 micrograms per cubic meter in October, when the winter heating season kicked off.

As a coastal city, Shanghai usually has mild to modest air pollution, but recent weather patterns have left the city's air stagnant.

Factory emissions in the adjacent provinces of Jiangsu and Zhejiang are among China's worst, according to the environmental group Greenpeace.

"Both Jiangsu and Zhejiang should act as soon as possible to set goals to reduce their coal consumption so that the Yantze River Delta will again be green with fresh air," Huang Wei, a Greenpeace project manager, said in a statement.