At least two million cars were taken off the road as Beijing rolled out its emergency response plan following its first-ever red alert for smog.

A grey haze hung heavy over China's capital on Tuesday after it announced the most severe smog warning under its four-tier system. The alert was active from 7.00 a.m. (local time) on Tuesday till noon on Thursday, Xinhua reported.

On Tuesday, the average density of PM2.5 exceeded 234 micrograms per cubic metre in the downtown areas.

Schools were closed, work at outdoor construction sites suspended, and factories have been ordered to halt operations. Beijing also closed several highway sections to reduce traffic.

The traffic ban, based on the odd or even license plate numbers, takes at least two million private cars off the road. In addition, 30 percent of government cars were subject to travel bans.

Beijing traffic authority worked through the night to ensure that the public transportation system can accommodate extra passengers on Tuesday.

Over 840 buses were added, and by 10.00 a.m. (local time) on Tuesday, 53 more subway trains were added across 15 lines.

Kindergartens, primary and high schools were advised to suspend classes, however, some teachers still went to school to support students who had chosen to brave the poor air and attend class.