Pedestrians walks in fog in Harbin, Northeast China's Heilongjiang province, Oct 21, 2013. [Photo/Xinhua]

HARBIN - A heavy smog shrouded Northeast China's Harbin city on Monday for the second straight day, forcing the closure of schools and highways.

The density of PM 2.5 - airborne particles measuring less than 2.5 microns in diameter, exceeded 500 micrograms per cubic meter on Monday morning, leading a visibility of less than 50 meters, in downtown Harbin, the capital of Heilongjiang province.

Meteorological authorities issued a red alert, the highest, for the heavy smog in several cities in the province at 9 am Monday, forecasting the smog will continue in the next 24 hours.

The Taiping International Airport in Harbin was also closed.

The city's education authorities have issued a notice for the suspension of classes for all primary and middle schools in Harbin.

Some public buses also stopped running due to the smog in Harbin. Many citizens had to walk or take other vehicles to work and were late.

Sunday marked the beginning of the heating season in Harbin, a key factor for the smoggy weather.