BEIJING, Feb. 21 -- More than 100 factories in Beijing will either halt or limit their activities as the Chinese capital raised its pollution alert on Friday amid signs of lingering smog over the weekend.

Beijing authorities have ordered 36 companies to halt production and another 75 to reduce production as part of a response mechanism triggered by an orange pollution alert, the second highest in a four-tier system.

Authorities cranked up the alert to orange a day after it put in place a yellow one when the city's Air Quality Index (AQI) exceeded 250, indicating serious air pollution.

The AQI measures six airborne pollutants, including PM 2.5, the particles with a diameter less than 2.5 microns that have been a major contributor to the smog shrouding much of north and east China in recent years.

AQI readings at various monitoring stations in downtown Beijing have topped 300 micrograms per cubic meter as of 3 p.m. on Friday, more than 10 times the level considered safe by the World Health Organization.

In addition to cutting industrial activity, the municipal government also urged citizens to stay indoors and use public transportation.