Air pollution readings in China's notoriously polluted capital were at dangerously high levels for the second straight day Saturday, with hazy skies blocking visibility and authorities urging people to stay indoors.

Local officials warned that the severe pollution in Beijing — reportedly the worst since the local government began collecting data a year ago — was likely to continue until Tuesday.

The Beijing Municipal Environmental Monitoring Center said on its website that the density of PM2.5 particulates had reached 700 micrograms per cubic meter in many parts of the city, a level considered extremely hazardous.

By 5 p.m., the center's real-time reporting showed the air quality indexes at or approaching the maximum 500 from some monitoring stations. The index runs from zero to 500 and accounts for the level of airborne PM 2.5 particulates — tiny particles considered the most harmful to health.

Generally, the air quality is considered good when the index is at 50 or below, and hazardous with an index between 301 and 500, when people are warned to avoid outdoor physical activities. Those with respiratory problems, the elderly and children are asked to stay indoors.

The air started to turn bad on Thursday, and monitors in Beijing reported air quality indexes above 300 by Friday. The monitoring center said Saturday that the pollution was expected to linger for the next three days and urged residents, especially the most vulnerable, to stay home as much as possible.

The air quality data are the worst in Beijing since the municipal government began to track PM2.5 early last year, said Zhou Rong of the environmental organization Green Peace.

Monitors at the U.S. Embassy in Beijing recorded an off-the-chart air-quality reading of 755 at 8 p.m. Saturday and said the PM2.5 density had reached 886 micrograms per cubic meter. It was unclear whether the embassy's data were the worst since it began collecting and sharing such information in 2008.

Readings are often different in different parts of Beijing. Chinese authorities and the United States also have different ways to calculate the air quality index, although their indexes are "highly similar" at the two ends of the spectrum, said Ma Jun, founder of the nongovernmental Institute of Public and Environmental Affairs in Beijing.

Air pollution is a major problem in China due to the country's rapid pace of industrialization, reliance on coal power, explosive growth in car ownership and disregard to environmental laws. It typically gets worse in the winter because of heating needs.

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In Beijing, authorities have blamed foggy conditions and a lack of wind for the high concentration ofair pollutants.

Several other cities, including Tianjin on the coast east of Beijing and southern China's Wuhan city, also reported severe pollution over the last several days.