Beijing is experiencing its worst air pollution this year, with monitoring sites throughout the city reporting extremely hazardous levels of pollutants.
City authorities issued a rare orange alert, the second highest of four danger levels. Schools suspended outdoor activities and polluting factories were required to reduce production.
The city reported the level of tiny PM2.5 particles in the air to be more than 600 micrograms per cubic metre late on Monday.
The US Embassy in Beijing reported the level to be 625 micrograms per cubic metre at 5pm local time.
The World Health Organisation considers the safe level to be 25 micrograms per cubic meter on a 24-hour average basis.
Visibility was cut to several hundred metres as buildings receded into thick smog. People complained of a smoky, pungent odour, and many wore tight-fitting face masks.
"I felt like my lungs were blocked," said Xu Pengfei, a security guard at an office building in downtown Beijing. "We have to stand in the open for many hours a day, and the pollution really affects us."
Liu Juntang, a businessman whose company develops environmental technology, said face masks could not give protection from the tiny pollutants on days of extremely heavy pollution.
"The air pollution is all-encompassing, and it requires both the government and individuals to shoulder the responsibility to clean up the air," he said.
Beijing has vowed to clean up its notoriously foul air and had been doing fairly well this year, with generally cleaner air than in 2014.
But the city was hit by unusually cold days and early snow in November, and winter heating was turned on. For the entire month, the capital saw hardly any blue skies and was shrouded in persistent smog.
The air quality worsened on Friday and continued to deteriorate throughout the weekend, prompting the city government to issue an orange alert on Sunday. Authorities said they did not issue the highest red alert because they were expecting the air quality to improve by Wednesday.
The last time Beijing issued an orange alert was in February 2014.