Blanket of smog covers Chinese city of Harbin

The Chinese city of Harbin has been blanketed by clouds of choking pollution, slashing visibility to a few metres, shutting schools and halting transport in scenes that underscored the nation's environmental challenges.

Elementary and middle schools were ordered to cancel class and operations were halted for public buses, long-distance coaches and the airport, CCTV reports said. Highways were also shut, although multiple-car crashes were still reported.

The smog in Harbin, a city of more than 10 million people in far northeastern China, came as it activated its public heating system in advance of the frigid winter, according to local media.

Figures from monitoring stations in central Harbin showed concentrations of PM2.5 - tiny airborne particles considered the most harmful to health - reached 1,000 microgra per cubic metre, 40 times the World Health Organisation's recommended standard.

They began to fall in the early evening but the overall air quality index, a different measure, was still being given as 500, the maximum level on the Chinese scale, and described as "beyond index".

The extremely polluted air came days after the government in Beijing announced that it plans to reinstate an odd-even car ban for days when the city's air pollution reaches red-alert levels.

Under the plan, which China's capital city employed during the 2008 Olympics and again in 2011, cars will only be allowed to drive on alternating days when severe air pollution persists for three or more days.