BEIJING, Feb. 24 -- Beijing on Monday maintained its orange pollution alert, the second-highest alert level, as severe smog is forecast to linger in the Chinese capital for another three days.

The density of PM2.5, particles smaller than 2.5 microns in diameter, which have been a major contributor to the smog, climbed again Sunday night and remained at high levels, the city's office for severe air pollution emergency response said in a statement.

The agency advised children and the elderly to stay indoors and to wear masks while going out. It also urged residents to take public transport and reduce driving. The agency urged middle schools, primary schools and kindergartens to reduce outdoor activities.

As students returned to schools on Monday, outdoor physical education classes and routine national flag raising ceremonies were canceled. Classes will not be suspended unless a higher red alert is issued.

Liu Wei, vice president of Cuiwei Primary School, said school authorities have ordered the cancellation of all outdoor activities and to keep students indoors.

A teacher at Zhongguancun No. 3 Primary School told Xinhua that the school has canceled the routine flag-raising ceremony and shifted physical education classes indoors.

City authorities on Friday ordered 36 companies to halt production and another 75 to reduce production as part of a response mechanism when the pollution alert was raised to orange from yellow.

Beijing issued a yellow pollution alert on Thursday, the first since last October, when the emergency response system was put into place.

Beijing has a four-tier alert system, with blue, yellow, orange and red indicating the air pollution level in order of increasing severity.