Beijing and about two dozen cities saw the year's worst smog on Saturday, which may last for days, making pollution worse in a country where 1.2 million die of it annually.
At least 24 cities are already under red alert -- highest of the four-tier warning system for pollution.
During the alert, construction activities are suspended and schools and kindergartens are shut. The cars follow the odd-even license number plate rule.
Beijing activated its first red alert for smog this winter as the air began to turn hazy on Friday, ending several days of blue sky.
All road construction and maintenance sites have been ordered to stop work, Xinhua news agency reported.
At 1 p.m. Saturday, the PM2.5 density exceeded 200 in the downtown areas of the capital, indicating the air is heavily polluted, according to the Beijing Municipal Environmental Monitoring Centre.
"It's killing and it's already dark at 4 p.m.," a resident of Beijing told IANS on the phone.
Beijing's environmental and housing authorities have strengthened inspection of vehicles, construction sites and factories, punishing those that fail to meet the requirements under the red alert.
The air quality index (AQI) readings at some sites in Tianjin, a northern port city neighbouring Beijing, exceeded 300 on Saturday morning, indicating serious air pollution.
Tianjin dispatched government inspection teams to check whether response measures have been implemented by companies, construction sites and drivers.
Air pollution is one if the biggest problems of China, which has taken steps in this regard.
Many top officials in provincial governments have been sacked for failing to contain pollution.
In 2015, China shut down some 17,000 polluting units in the country.
