People in Beijing were warned to stay indoors Wednesday as the capital was shrouded in yellow smog with pollution reaching dangerous levels.

``Under these polluted conditions, we propose that the majority of citizens reduce their time outdoors and avoid breathing this seriously polluted air,'' said the Beijing Environmental Protection Bureau in a warning posted on its Web site.

Beijing's air quality has been at the lowest level for the past two days with the air ``seriously polluted,'' the bureau said.

Experts said the capital was experiencing a heat inversion, where warmer air in the atmosphere was keeping the colder ground air in place.

This was making it difficult for the pollution to disperse.

Meanwhile, warmer spring temperatures in the capital also meant that work at construction sites has increased, further kicking up dust that is mixing with the ever-increasing pollution from cars.

``Under this situation, the thickness of every kind of air pollution has clearly increased, especially breathable suspended particles, which have risen rather quickly,'' the bureau said.

Suspended particle levels were hovering around the dangerous level of 400 and 500 micrograms per cubic meter, it said.

According to satellite photos, the inversion was lingering over northern Hebei and Shanxi provinces and extending southward to the Yangtze river, the paper said.

During the past decade of China's economic boom, the nation has also produced some of the world's most polluted cities, with Beijing's air quality regularly ranking among the worst in the country and world.

AGENCE FRANCE-PRESSE