Beijing authorities have issued a second pollution red alert, little more than a week after the first ever such warning.
China’s capital will see hazardous smog from December 19 until December 22, the Beijing Meteorological Service said.
Nationwide, a vast area from Xian in central China to Harbin in the north-east would also be affected, the National Meteorological Center said.
The red alert triggers restrictions on vehicle use, factories and construction work.
The government has promised to take action to address often dangerous levels of pollution.
Meteorological authorities have said that the regional smog is likely to be worse than the last red alert earlier this month, with the PM2.5 pollution level to exceed 500 micrograms per cubic meter.
The smog which hit Beijing on December 8 had peaked just below 300. Residents are encouraged to stay indoors if levels exceed that level.
The World Health Organization (WHO) recommends 25 micrograms per cubic meter as the maximum safe level.
Authorities released a map showing that heavy smog would blanket a swathe of China spanning nearly 1,200 miles, encompassing at least 12 major cities, with Beijing and nearby city Shijiazhuang heaviest hit.
The other cities would experience medium or lesser levels of smog.
The news was greeted with exasperation and worry among Chinese citizens online.
The current four-level alert system was instituted about two years ago, although the red alert had never been issued until this month.
Coal-powered industries and heating systems – in heavy use during the cold Beijing winter – are major contributors to the smog, which is made worse by weather conditions and the city’s geography.
Beijing is bordered to the south and east by industrial areas that generate pollution, and to the north and west by mountains that trap it over the city.
China still depends on coal for more than 60% of its power, despite big investments in renewable energy sources.
Earlier this month China was part of the landmark Paris climate change agreement that set a course for China, and the world, to move away from fossil fuels in the long term.
Beijing authorities have decided to close schools and to stop outdoor construction after the Chinese capital issued its first “red alert” over smog levels.
The red alert is the highest possible, and has not been used in Beijing before, the state-run Xinhua news agency says.
Chinese authorities expect more than three consecutive days of severe smog.
Cars with odd and even number plates will be banned from driving on alternate days.
The alert comes as China, the world’s largest polluter, takes part in talks on carbon emissions in Paris.
Current pollution levels in Beijing are actually lower than last week’s, but the red alert has been placed because of levels expected over the coming days.
The order will last from 07:00 local time on December 8 until 12:00 on December 10, when a cold front is expected to arrive and clear the smog.
China’s CCTV news channel reported at the weekend that some parts of Beijing had visibility of only 660ft.
Coal-powered industries and heating systems, as well as vehicle emissions and dust from construction sites, all contribute to the smog which has been exacerbated by humidity and a lack of wind.
At 18:00 local time on December 7, the air pollution monitor operated by the US Embassy in Beijing reported that the intensity of the poisonous, tiny particles of PM 2.5 was 10 times above the recommended limit.
The level in Beijing reached more than 256 micrograms per cubic meter in some of the worst-affected areas. The World Health Organization considers 25 micrograms per cubic meter to be a safe level.
Activists said the level hit 1,400 micrograms per cubic meter in the north-east city of Shenyang last month, saying it was the worst seen in China.
Last week, activists from Greenpeace had urged the Chinese government to declare a red alert. Another Chinese city, Nanjing, issued a red alert in December 2013.
On November 30, Beijing issued an orange alert – the second-highest of the four-tier system adopted in 2013.
Correspondents say Chinese officials had been unwilling to commit to hard targets on reducing carbon emissions, but have now realized that the dependence on fossil fuels has to stop.
Chinese President Xi Jinping vowed to take action on the emissions at the current global climate change talks in Paris.
McDonald’s China fries supplier got Beijing’s biggest ever pollution fine, state media report.
Beijing Simplot Food Processing will have to pay 3.9 million yuan ($629,000) after its waste water was found to have levels of impurities above legal limits.
The company said it also serves other companies in East Asia.
IBeijing Simplot Food Processing said it had paid the fine and would take steps to avoid a recurrence.
McDonald’s said in a statement that it took the issue “very seriously” and would be monitoring Beijing Simplot’s compliance. It said it suppliers “must comply with all relevant local laws and regulations”.
A small amount of polluted water had entered the city’s main water pipes, but was not dangerous to the public, a local official was quoted as saying by Jinghua Daily News.
China has been facing international pressure to clean up its environment and an increasing number of local protests concerning pollution.