BEIJING (Reuters) - Choking smog all but shut down one of northeastern China's largest cities on Monday, forcing schools to suspended classes, snarling traffic and closing the airport, in the country's first major air pollution crisis of the winter.

An index measuring PM2.5, or particulate matter with a diameter of 2.5 micrometers (PM2.5), reached a reading of 1,000 in some parts of Harbin, the gritty capital of northeastern Heilongjiang province and home to some 11 million people.

A level above 300 is considered hazardous, while the World Health Organisation recommends a daily level of no more than 20.

The smog not only forced all primary and middle schools to suspend classes, but shut the airport and some public bus routes, the official Xinhua news agency reported, blaming the emergency on the first day of the heating being turned on in the city for winter. Visibility was reportedly reduced to 10 meters.

The smog is expected to continue for the next 24 hours.

Air quality in Chinese cities is of increasing concern to China's stability-obsessed leadership because it plays into popular resentment over political privilege and rising inequality in the world's second-largest economy.

Domestic media have run stories describing the expensive air purifiers government officials enjoy in their homes and offices, alongside reports of special organic farms so cadres need not risk suffering from recurring food safety scandals.

The government has announced plans over the years to tackle the pollution problem but has made little apparent progress.

Users of China's popular Twitter-like Sina Weibo microblogging site reacted with both anger and bitter sarcasm over Harbin's air pollution.

"After years of effort, the wise and hard-working people of Harbin have finally managed to skip both the middle-class society and the communist society stages, and have now entered a fairyland society!" wrote on user.

Other parts of northeastern China also experienced severe smog, including Tangshan, two hours east of Beijing, and Changchun, the capital of Jilin province which borders Heilongjiang.

Last week, Beijing city released a color-coded alert system for handling air pollution emergencies, to include the temporary halt of construction, factory production, outdoor barbeques and the setting off of fireworks.

Beijing suffered its own smog emergency last winter when the PM2.5 surpassed 900 on one particularly bad day in January.

(Reporting By Adam Rose; Editing by Ben Blanchard and Michael Perry)