Beijing is struck by a sandstorm on May 4 (photo from Chinanews.com)

BEIJING, May 4 (Xinhua) -- A sandstorm swept over large swathes of north China including Beijing on Thursday, turning the sky yellow and obscuring visibility.

The city's meteorological center issued a blue alert for sandstorm Thursday morning, forecasting winds carrying sand and dust across the capital. Many pedestrians in downtown Beijing were seen wearing protective masks.

Most monitoring stations in the city showed PM10 readings of more than 1,000 micrograms per cubic meter as of 4 a.m. Thursday, according to data from Beijing Municipal Environmental Monitoring Center.

Visibility plummeted to as low as one kilometer in many parts of Beijing and it is expected to reduce further.

Parts of Beijing, as well as Gansu, Hebei, Heilongjiang, Jilin, Shaanxi and Shanxi provinces and the autonomous regions of Inner Mongolia, Ningxia and Xinjiang will see sandstorms from Thursday to Friday, said the National Meteorological Center.

Zhu Jiang, head of the Institute of Atmospheric Physics of the Chinese Academy of Sciences, said the sandstorms originated in Mongolia.

China has a four-tier color-coded system for severe weather, with red being the most serious, followed by orange, yellow and blue.