Choking clouds of pollution blanketed a Chinese city famed for its annual ice festival Monday, slashing visibility to a few metres, shutting schools and halting transport in scenes that underscored the nation's environmental challenges.
Footage from Harbin on state broadcaster CCTV showed a screen full of charcoal-brown smog, with faint shapes and colours beneath hinting at roads, cars and traffic signals.
Elementary and middle schools were ordered to cancel class and operations were halted for public buses, long-distance coaches and the airport, reports said.
Highways were also shut, although multiple-car crashes were still reported.
Drivers who jumped red lights because they could not see them "will not be penalised", the state news agency Xinhua quoted local traffic official Xue Yuqing as saying.
Visibility in the city centre would drop to less than 50 metres (160 feet), it added.
"How scary! It's the apocalypse!" one Internet user in China wrote about the air quality in Harbin, which was a hotly-discussed topic online. Some users compared images of the pollution to scenes from a horror movie.
Figures from monitoring stations in central Harbin showed concentrations of PM2.5 -- tiny airborne particles considered the most harmful to health -- reached 1,000 micrograms per cubic metre, 40 times the World Health Organisation's recommended standard.
They began to fall in the early evening but the overall air quality index, a different measure, was still being given as 500, the maximum level on the Chinese scale, and described as "beyond index".
The smog in Harbin, a city of more than 10 million people in far northeastern China, came as it activated its public heating system ahead of the frigid winter, the Beijing Times said.
A top-level red alert for "thick smog or fog" was also issued for the entire province of Heilongjiang, which has Harbin as its capital, along with nearby Jilin and Liaoning, Xinhua said.
It came days after the government in Beijing announced that it plans to reinstate an odd-even car ban for days when the city's air pollution reaches red-alert levels.
Under the plan, which China's capital city employed during the 2008 Olympics and again in 2011, cars will only be allowed to drive on alternating days when severe air pollution persists for three or more days.
Pollution from rapid development and heavy coal use plagues wide swathes of China, prompting public criticism and pledges from the country's new leadership to make improvements.
A thick smog that covered the capital Beijing earlier this year -- with similar PM2.5 levels as Harbin on Monday -- made global headlines.
At the time Harbin escaped the very worst of the pollution, but huge swathes of northern China have been blanketed by smog at various times this year.
The State Council, or cabinet, said in June that among other measures it would hold local officials accountable for improving air quality.
Air pollution contributed to 1.2 million premature deaths and 25 million healthy years of life lost in China in 2010, the US-based Health Effects Institute said in March.