The CCTV headquarters is shrouded in heavy smog in the central business district in Beijing, China Dec 7, 2015.[Photo/Xinhua]

BEIJING - Beijing has issued its first red alert for air pollution under a four-tier emergency response system created in October 2013.

The red alert, the most serious level, will last from 7 a.m. on Tuesday to noon on Thursday. The city issued an orange alert, the second-highest level, on Saturday.

Smartphone air quality apps, popular among Beijingers, showed an Air Quality Index (AQI) reading of more than 200 for almost all monitoring stations across the city on Monday, advising people to reduce outdoor activities.

It comes after the worst period of air pollution this year took AQI readings in the capital to the maximum 500 on Nov. 30.

While the city did not issue a red alert during that spell of smog, the rules drawn up two years ago state that a red alert should be issued in periods when the AQI is expected to surpass 200 for three consecutive days.

Under a red alert, the city's emergency management headquarters has advised kindergartens, primary and high schools to suspend classes, banned outdoor operations on construction sites and required some industrial plants to limit or stop production.

Few students showed up to Beijing No.2 Experimental Primary School on Monday as the school sent out text messages giving them the choice of either staying home or coming to school

He Jinglong, a physical education teacher of the affiliated school of Beijing Jiaotong University, led students in indoor exercise on Monday.

"We designed a set of exercises specially for smoggy weather to keep students fit," said He.

The Beijing Education Commission issued a notice on Monday night asking all middle, primary schools and kindergartens to suspend classes during the red alert period.

Car use will be limited during the red alert period as cars are allowed on the roads on alternating days depending on the odd or even numbers of their license plates. In addition, 30 percent of government cars will be banned from streets on an odd/even basis.

The restriction of car use is expected to push an extra two million passengers onto public transport per day. Beijing will add 21,000 to 25,000 buses, including 8,182 clean-energy buses, to the roads from Tuesday.

According to the Beijing Municipal Environmental Monitoring Center, heavy smog will linger until Thursday. It is expected to disperse on Thursday afternoon as a cold front arrives.