China has issued its highest-level smog alert this year, with monitors reporting levels of pollutants 22 times those recommended by the World Health Organization.
To help reduce the smog, authorities issued orders on Monday to temporarily shut down or reduce production at factories in Hebei, a neighbouring province.
"Hebei is the epicentre of China's pollution problem," Al Jazeera's Adrian Brown reported from Beijing.
Classes have also been suspended while heavy lorries have been ordered off the roads.
In Pictures: Worst smog of the winter envelopes Beijing
As of 07:00 GMT on Monday, US embassy air-quality monitors in Beijing reported a pollution index of 568.
Beijing residents have also been advised to stay indoors as visibility across the city also fell.
WHO considers the safe level of the air at 25 micrograms per cubic metre of the particulates. Pollution levels over 100 are already considered dangerous.
Authorities blame coal burning for winter heating as a major culprit for the air pollution.
Meanwhile, a similar situation has been reported in Shanghai, where hazardous smog has engulfed much of the city's skyline, and with the smog index at 171 as of 02:00 GMT.
Al Jazeera's Brown said the situation in Beijing and Shanghai highlighted the pollution problem in China just as the UN summit on climate change in Paris was getting under way.
"Seeing is believing," he said against a background of smog-filled Beijing skyline.
OPINION: The right price for preserving our climate
"It's ironic as President Xi Jinping has been basking in the limelight in Paris, his capital city Beijing has been smothered in pollution today."
China is considered to be the biggest producer of emissions in the world, generating as much as 6,018 million tonnes of greenhouse gases a year.
While China has taken some action to reduce pollution, including investing at least $115bn in renewable energy, it has still got a long way to go in reducing pollution, Al Jazeera's Brown said.
He said that whatever is agreed in Paris will not provide an immediate solution to Beijing's problem.
More than 150 world leaders are converging on Paris on Monday to start talks on a deal meant to limit man-made emissions and avoid the worst effects of climate change.
Negotiators are expected to continue hammering out the details of the agreement until December 11, after an initial push from world leaders to give momentum to a process that has often stopped short of delivering an international deal.
Some of the stumbling blocks that have frustrated past negotiations include the positions of the world's two largest emitters, China and the US.
US President Barack Obama has indicated a desire to reach an agreement despite opposition in Congress, and China has committed to limiting its carbon intensity by 2030.
Source: Al Jazeera and agencies