China's capital has been shrouded in dangerous smog, cutting visibility down to a few hundred metres.
A count of small particulate pollution reached more than 25 times recommended levels.
A grey haze filled the sky, leaving an industrial, burning smell hanging in the air.
The official air quality index reached the top of the scale at 500.
China's cities are often hit by heavy pollution, blamed on coal-burning by power stations and industry, as well as vehicle use.
It's become a major source of discontent with the ruling Communist Party.
"I can't believe how bad it is," said US tourist Richard Deutsch in Tiananmen Square in central Beijing, which was shrouded in heavy smog.
"I have never experienced air this bad.
"It's worse than LA.
"I feel kind of bad for the people, it's taking years off their life."
Authorities have become more open about pollution levels, in part as a response to public pressure.
But officials have implied that it will take years before the situation improves.
The pollution has been linked to hundreds of thousands of premature deaths, and has tarnished the image of Chinese cities including Beijing, which saw a 10 percent drop in tourist visits during most of 2013.
"There are less and less tourists coming to Beijing," said tourist guide Xiao Yan.
"It's normal as people keep on talking about the bad air quality on the internet."
China's State Council said last year that "concentrations of fine particles" in the capital's air would fall by approximately 25 percent from 2012 levels by 2017.
Other major Chinese cities on China's affluent east coast, including Shanghai and Guangzhou, will see reductions of between 10 and 20 percent from 2012 levels over the same period, it said.
China is the world's biggest coal consumer and is forecast to account for more than half of global demand this year.
SOURCE: Agence France-Presse(AFP)