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

Heavy smog shrouded northeast China on Monday, forcing the closure of schools, as well as disrupting flights and highways.

Meteorological authorities in Heilongjiang, Jilin and Liaoning provinces issued a red alert for thick smog or fog.

The fog will worsen in Harbin, Yichun, Daqing, Suihua, Jiamusi and Qitaihe cities in Heilongjiang with visibility of less than 100 meters in the next 24 hours, said the provincial meteorological station.

"After walking outdoors for a while, I felt it difficult to breathe," said Chu Yushu, a resident in Harbin, provincial capital of Heilongjiang, which had visibility of less than ten meters in parts of the downtown area.

The density of PM2.5 -- airborne particles measuring less than 2.5 microns in diameter, exceeded 500 micrograms per cubic meter early Monday in Harbin.

All highways in Heilongjiang and Taiping International Airport in Harbin were closed due to the weather. Harbin's education authorities issued a notice to suspend classes at all primary and middle schools.

Some public buses also stopped running in Harbin. Many citizens had to walk or use other means of transport to work.

Drug stores in the city were selling out of masks.

"The sale of masks was at least ten times the normal rate. We sold more than 140 masks this morning in our shop," said a salesman of a drug store in Xiangfang District, Harbin.

"My mask turned black after I wore it for half an hour," said a netizen in Harbin in his microblog.

Highways were closed and flights disrupted in Jilin and Liaoning.

"The building 50 meters away cannot be seen clearly. I smelt irritative coal smoke when I opened the window of my house," said Sun Qi, a resident in Changchun, provincial capital of Jilin.

"We were scared by such thick fog when we opened the door to go out. The air quality is so poor outside and the smell irritative. We have to stay at home," said 74-year-old Zhong Jinfeng in Changchun. Zhong and his wife usually take a walk each day.

The meteorological station in Changchun issued a red alert at 2:45 a.m. Monday. Many pedestrians and traffic police were wearing masks.

With visibility of less than 100 meters, all 22 flights in Changchun Longjia Airport were delayed due to the foggy weather. Visibility was less than 500 meters in most of Jilin.

"Many drivers cannot see my gestures because of the smog. I sometimes have to rely on shouting to let them know my guidance," said a traffic policeman surnamed Gao at a viaduct on Guangfu Road in Changchun.

The weather also delayed trains and caused the closure of most of the highways in Jilin.

Sunday was the beginning of the heating season in Harbin. Heating supply companies have also begun operation in Jilin.

The big pollutant emissions from coal-burning, vehicle exhaust and the burning of crop stalk on the outskirts and slight winds, were all factors leading to the smoggy weather, according to environmental authorities in Heilongjiang.

Air pollution has become one of the major public concerns in China after decades of rapid development. Cities, including the capital Beijing, have vowed to pour hefty investment to treat air pollution.

In September, the country announced a government action plan to tackle air pollution, pledging to cut coal use, shut down polluters and promote cleaner production. The plan also aims to cut the density of inhalable particulate matter by at least 10 percent in major cities nationwide by 2017.