Updated

Air pollution has reached dangerously high levels in China's capital, where the concentration of toxic small particles registered more than two dozen times the level considered safe.

The density of PM2.5 particles was at about 350 to 500 micrograms per cubic meter in the midmorning, and had reached as high as 671 at 4 a.m. at a monitoring post at the U.S. Embassy in Beijing. That is about 26 times as high as the 25 micrograms considered safe by the World Health Organization.

Serious air pollution plagues most major Chinese cities, where environmental protection has been long sacrificed for the sake of economic development. In recent years, China has beefed up regulations to fight pollution.

Beijing authorities say the severe pollution is likely to continue through Friday.