SHANGHAI - Smog shrouded the skyline of China's financial hub Shanghai as air pollution in the city hit its worst level on Monday.

Air quality in Shanghai was recorded at 303 on the official Air Quality Index (AQI) on Monday morning, crossing the 300 threshold that indicates the most severe level of air pollution on the chart.

The AQI monitors six pollutants on an hourly basis, including PM 2.5, or particles measuring less than 2.5 microns in diameters, which has been widely blamed for the worsening air pollution across China in recent years.

Shanghai's environmental watchdog listed PM 2.5 as the major pollutant on Monday, without disclosing detailed readings.

The World Health Organization sets a daily guideline value for PM 2.5 at 25 micrograms per cubic meter, while it has been around 10 times this volume in Shanghai in recent days.

Shanghai saw deteriorating air quality over the weekend. The AQI index rose above 230 on Sunday, when the city held its annual Shanghai International Marathon.

The PM 2.5 reading also hit 248 micrograms per cubic meter on that day, according to the Shanghai Environmental Protection Bureau.

The marathon took place as usual despite the heavy pollution. Some participants wore masks.

The dangerous level of pollution has prompted Shanghai authorities to curb industrial production and activities at construction sites and docks that may add dust to the air. Citizens are advised to stay indoors.