The Chinese capital dawned with blue skies and less-polluted haze due to cold air from the north, but forecasts indicated that the smog will return to significant levels on Tuesday and Wednesday, Efe news reported.
The orange alert, the second most serious level on a scale of four, was initially announced from December 30 to January 1, but has now been extended until midnight on Wednesday.
On Sunday, the dense polluted fog that covered northeastern China in recent days forced 309 flights to be cancelled and all motorways in the port city Tianjin, east of Beijing, to be closed.
Some 126 flights at Beijing Capital International Airport were also cancelled.
Beijing on Sunday recorded the concentrations of more than 500 micrograms per cubic metre of PM 2.5 particles, the most harmful to health, which is 50 times more than the World Health Organisation's recommended levels.
A total of 24 Chinese cities started 2017 with a red alert - the most serious level - for heavy air pollution, and 21 others, including the capital and Tianjin, are on the orange alert, and have banned the most polluting vehicles from the road and stopped production in certain factories.
