China's capital issued its first-ever red alert for pollution yesterday, as a new blanket of choking smog was projected to descend on the city.

From today, half of Beijing's private cars will be ordered off the road, with an odd-even number plate system in force, and 30 percent of government vehicles also garaged.

Outdoor construction sites will cease operations, but only some industrial plants will have to "implement measures to limit or stop production", said Beijing's Environmental Protection Bureau on its verified social media account, adding that fireworks and barbecues were also banned.

"People should to the best of their ability reduce outdoor activities," it said. "If you are engaging in outdoor activities you should wear a mask or take other protective measures."

Kindergartens, primary and middle schools were urged to close, it added, without explicitly making the measures mandatory.