Visibility in Harbin, the capital of Heilongjiang province, was reduced to less than 50 meters (164 feet), prompting officials to issue a red alert, grounding all flights in the area, closing highways in Heilongjiang and shuttering Taiping International Airport.
And the smog was expected to worsen into Tuesday.
"After walking outdoors for a while, I felt it difficult to breathe," said Chu Yushu, a resident in Harbin.
"We were scared by such thick fog when we opened the door to go out," added Zhong Jinfeng, 74, who normally takes a walk with his wife each day. "The air quality is so poor outside and the smell irritative. We have to stay at home."
Drug stores noted they saw sale of face masks go through the roof as residents scrambled to protect themselves from the dangerous air.
Environmental authorities in Heilongjiang blamed coal-burning, vehicle exhaust, burning of crop stalks and winds in causing the heavy smog.
In September, China announced plans to combat severe pollution by cutting reliance on coal, promoting cleaner production and shutting down the worst offenders, with a goal of cutting inhalable particulate matter by 10 percent by 2017.
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