Beijing has issued a second pollution red alert, little more than a week after the first ever such warning.
The Chinese capital will see hazardous smog from Saturday until Tuesday, the official meteorological service said.
Nationwide, a vast area from Xian in central China to Harbin in the north-east would also be badly hit, foreign media reports.
Beijing authorities have advised residents to avoid outdoor activity and for schools to stop classes.
The red alert - the highest of a four-level alert system instituted two years years ago - also triggers restrictions on vehicle use, factories and construction work.
The government has promised to take action to address often dangerous levels of pollution.