Two photos show the Palace Museum under the clear sky on December 4, and same building obscured by heavy smog on December 7, 2015 in Beijing. [Photo: Chinanews.com]

At least two million private cars were taken off the road in the Chinese capital as Beijing rolled out its emergency response plan following its first-ever red alert for air pollution.

The alert remains active until noon on Thursday.

Under the alert, schools have been closed, work at outdoor construction sites suspended, and factories have been ordered to halt operations.

The city has also closed several highway sections to reduce traffic.

The traffic ban, based on odd or even license plate numbers, will take at least two million private cars off the road.

840 buses and 53 subway trains have been added to ensure that the public transportation system can accommodate extra passengers.

Li Hui, deputy head of Sihui subway station, says they have also taken some measures to cope with the commuter increase.

"Since the primary and middle schools all closed and enterprises have adopted flexible work days, the passenger flow pressures on us at rush hours have been effectively alleviated, or are not as much as expected. As for the transport capacity, we have added temporary subway trains, and normally now we have one train coming every two minutes."

Meanwhile, the Ministry of Environmental Protection has demanded local environmental authorities punish enterprises that fail to launch emergency response plans to cope with the smog.

The ministry found some polluters continued to operate after teams were dispatched to areas around Beijing to ensure emergency plans were in place.

Zheng Zaihong is with the Beijing Municipal Environmental Protection Bureau.

"The operation of some enterprises should be suspended if conditions are allowed, but if not, temporary control measures should be taken to lower or limit production."

In neighboring Hebei Province, the environmental protection authority detected a dozen coal-burning enterprises operating illegally.

Inspection teams discovered that some other sources of air pollution, such as road and construction site dust, remained severe in central China's Henan Province and the eastern province of Shandong.