The weather observatory upgraded its fog alert from yellow to orange for Saturday evening to Sunday morning under the country's four-tier warning system, with red as the most severe, followed by orange, yellow and blue.
Fog will be thickest in Hebei, Shandong and Henan provinces with visibility less than 200 metres, according to the National Meteorological Centre.
Visibility will be less than 500 metres in Beijing, Tianjin, Jiangsu, Anhui, Hubei and Hunan, weather observatory said, state-run Xinhua news agency reported.
The public has been warned to take protective measures when engaging in outdoor activities, and those who suffer from respiratory diseases are advised to stay indoors.
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Earlier, the Centre had issued an yellow alert for Beijing and several other cities for pollution as smog returned to haunt the Chinese in the New Year. The new alert was issued in Beijing, Tianjin, Hebei cities, and Shanxi, Henan, Shandong, Shaanxi, Anhui, Jiangsu and Sichuan provinces.
The Centre for the first time sounded red alert for smog on December 7, leading to a host of measures including shutting down schools and regulating traffic with odd and even numbers.
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In the last a couple of days, a total of 25 cities in China have issued red alerts for smog.
Meanwhile, a number of companies were found to have committed irregularities, harming China's environment protection efforts, the Ministry of Environmental Protection has said.
The irregularities, including unauthorised resumption of production despite a government ban and non-compliance with emission reduction measures, were discovered by 10 inspection teams recently dispatched to local regions by the ministry.