Beijing residents woke up to a white Christmas Friday morning but with the sky obscured by thick toxic smog rather than snow after more than 100 million people across China had been warned to stay indoors.

The capital and surrounding parts of northern China are regularly blanketed in deadly pollution associated with heavy industry and an increase in coal consumption during the winter months.

Counts of PM2.5 -- harmful microscopic particles that penetrate deep into the lungs -- in Beijing peaked at 620 micrograms per cubic metre as of early Friday, according to data from the US embassy.

The World Health Organization's recommended maximum exposure is 25 micrograms over a 24-hour period.

"If this only happened a few days a year, I'd put up with it, but it's paralysing for it to be like this every day!" said one angry social media user on China's Twitter-like Weibo platform. "Is this the new normal?"

The city posted contradictory alerts for Christmas day, with the Beijing Meteorological Service issuing an orange alert - requiring factories to limit expelled pollutants and schools to cease outdoor activities -- while the Beijing Municipal Environmental Protection Bureau issued only a yellow alert, the second lowest in a four-tiered, colour-coded warning system.

The smog is expected to last until Saturday afternoon.

Photos on social media showed empty freeways shrouded in a white-out of haze as authorities shut down highways in the region and grounded flights out of the capital.

More than 500 international and domestic arrivals and departures at Beijing's main airport were cancelled due to "heavy fog and low visibility", its website said.

"In this kind of weather, can Santa even find Beijing?" a user wrote on Sina Weibo.

In the neighbouring city of Tianjin, no fewer than 19 freeways were closed early Friday morning, according to the Tianjin Freeway Management Office's official microblog, with CCTV television reporting visibility of less than five metres (15 feet).

"Santa can't even get to Tianjin! I was wondering why my stocking had no gifts in it! Smog you better get out of here!" said another Weibo commentor.

Beijing issued its first-ever red alert on December 7, declaring emergency pollution measures following scathing public criticism of the city's weak response to choking smog that settled on the capital earlier in the month.

Its second and latest red alert, which lasted four days, concluded Tuesday night.

rld/bdh/mtp

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