Smog hovered over Beijing as limits on cars, factories and construction remained in force after the city imposed its first red alert for air pollution.

Cars with even-numbered licence plates were kept off roads, and schools and building sites stayed closed on the second of three days of restrictions triggered by the alert.

Far fewer than usual pedestrians walked the streets - many of them wearing white masks to filter the air.

Pollution levels at midday on Wednesday in downtown Beijing were mostly between 250 and 300 on the city's air quality index - suggesting the restrictions were having an effect.

The city's alert was triggered by a forecast on Monday that pollution levels would be above 300 for three days straight.

The index is strongly linked to levels of the dangerous tiny particles PM2.5, which at midday ranged from about 200 to 250 micrograms per cubic metre in the downtown area - eight to 10 times the safe level recommended by the World Health Organisation (WHO).

Insurance and water purifier saleswoman Zhang Jingtie said she had no choice but to be out despite restrictions through to Thursday that have prompted some businesses to close and some others to allow employees to work from home.