BEIJING, Dec. 8 -- 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 smog.

A gray haze hung heavy over Beijing on Tuesday, after it announced the most severe smog warning under its four-tier system. The alert is active from 7 a.m. Tuesday until noon on Thursday.

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

Under the alert, schools have been 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, will take at least two million private cars off the road. In addition, 30 percent of government cars are subject to travel bans.

Beijing traffic authority worked through the night to ensure that the public transportation system can accommodate extra passengers on Tuesday. More than 840 buses have been added, and by 10 a.m. 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.

While some students were happy for an excuse to skip school, others thought the smog was not to be celebrated.

"I'd rather go to school on a nice day than stay at home to protect myself from the filthy air," said Li Yue, a fifth-grade student at Cuiwei Elementary School in western Beijing.