Harbin City, capital of China's Heilongjiang province of 11 million people, has been enveloped by a chockfull of smog so thick authorities have been forced to cancel flights and suspend classes. Residents are scared out of their wits as the smog, expected to last for at least the next 24 hours, could become recurring as the country enters the winter season.

"How scary! It's the apocalypse!" ABC News quoted a Chinese Internet user as saying. Visibility in Harbin City dropped to less than 50 metres.

"I couldn't see anything outside the window of my apartment, and I thought it was snowing," Wu Kai, a housewife and mother, told AP.

"Then I realised it wasn't snow. I have not seen the sun for a long time."

On Monday, the index measuring PM2.5 in Harbin City reached a reading of 1,000, which is way double above the considered hazardous level of 300. The World Health Organisation had time and again reiterated that the daily safe level should not exceed more than 20.

The official state Xinhua news agency reported Harbin's smog forced the suspension of classes in all primary and middle schools. It likewise cancelled the operations as the airport as well as affected some public bus routes. At least 40 flights were affected on Monday morning.

Xinhua said the emergency environmental crisis was triggered as the residents turned on their heaters for the first time yet again owing to the winter season.

"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," Xinhua news agency reported, quoting environmental authorities in Heilongjiang.

The smog was so dense that "you can hear the person you are talking to, but not see him," the New York Times quoted a Sina Weibo user who happened to be a resident of Harbin.

Harbin City's smog has also affected other parts of northeastern China, including Tangshan and Changchun. Tangshan is two hours away located east of Beijing, while Changchun, capital of Jilin province, borders Heilongjiang province.