A photo shows Beijing is beleaguered by heavy smog on December 1, 2015. [Photo: 21cn.com]

Beijing has ordered all schools to keep students indoors and adjust classroom activities as heavy smog smothers the city.

On Tuesday, Beijing No. 2 Experimental Primary School allowed students to choose whether to study at home or in school while Beijing Digital School is running classes online, according to the city Commission of Education.

The smog worsened in north China on Monday and the meteorological center upgraded its alert from yellow to orange, the second most serious level.

Highways in Beijing and Henan Province have been temporarily closed due to low visibility.

The smog will continue in most parts of north China on Tuesday and Wednesday, the National Meteorological Center (NMC) forecast.

China has a four-tier color-coded weather warning system, with red representing the most severe weather, followed by orange, yellow and blue.