"Residents should take measures to protect themselves in order not to breathe heavily polluted air."

During the 24-hour period from Sunday noon, Beijing's air quality was rated a "hazardous" four on a scale of five, the bureau said, with five representing the worst level of pollution.

Besides the regular car pollution, the bad air quality was also due to the city's 6,000 coal-fire heating furnaces and up to two million home coal burners that went into operation this month to provide the capital with heat, it said.

Meanwhile the blanket of smog caused traffic chaos as drivers were left lined up at numerous highways that were shut down due to the atmospheric conditions.

Five highways in the capital and eight in neighboring Tianjin were shut down beginning late Sunday due to the fog, which reduced visibility in some places to as low as 10 meters (33 feet), Xinhua news agency reported.

During Beijing's early morning rush hour Monday, 154 accidents were reported, with some of the incidents due to heavy smog, the report said.

Fog was worse in northeastern Liaoning province and eastern Shandong province where air flights in and out of the regions were delayed since Sunday, separate reports said.

Beijing has set an ambitious annual target of realizing "blue sky days" of level two air quality or better 65 percent of the time.

As of November 14, the city had registered 213 blue sky days and needed an additional 25 more clean air days by the end of the year to reach the target, the environmental bureau said.

"We still have hopes to fulfill this year's task, it can be done but it will be a serious challenge and we must not take this lightly," it said in a report.

Beijing, a city of 15 million people, is regarded as one of the most heavily polluted in China, although authorities are trying desperately to improve the situation ahead of the 2008 Olympic Games.

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