BEIJING — A thick layer of deep gray smog swept into Beijing on Friday afternoon, bringing what the authorities said would be five days of the worst air pollution in a year.

The city issued its first red alert for air pollution of 2016, the most severe notice in a four-tier system, requiring schools to close and half of all privately owned cars to stay off the roads.

As lead-colored air blanketed the capital, moving in from the industrial heartland of surrounding Hebei Province, residents posted photographs on social media showing how the skies had changed to gloomy gray at sunset from bright blue in the morning. Some people posted online maps showing Japan and South Korea with clean air readings.

Red emergency signs on Beijing’s main roads alerted drivers to the smoggy conditions, and workers wore protective masks for their journeys home.