Beijing authorities stepped up their health warnings to residents on Wednesday as thick smog blanketed the Chinese capital and large swathes of the country for a third consecutive day.

The city's 20 million people were urged to shut windows, eat a "balanced diet" and drink plenty of water, while the municipal government advised the elderly, young and those with health problems to stay indoors or wear protective masks if they ventured out.

The warnings, on Internet news sites and microblogs and in regular bulletins by state broadcaster China Central Television, were issued as emergency measures were announced to counter the pollution.

Authorities announced the closure of 103 factories and ordered 30 percent of official cars off the road on Tuesday.

Meanwhile, a campaign for clean air legislation by real estate tycoon and Internet blogger Pan Shiyi -- who has 14 million followers on Sina Weibo, China's version of Twitter -- was gathering pace.

By Wednesday afternoon more than 43,000 people had voted in favour of new laws to tackle the smog in a survey posted by Pan, the chairman of Soho China, and a member of China's rubber-stamp parliament.

Pan spearheaded a campaign in 2011 to force Beijing to release transparent details on levels of tiny air particles known as PM2.5.

Reform-minded investor Xue Manzi, who has 10 million followers on Weibo, backed his campaign.

It was the fourth serious bout of toxic air in recent weeks. The winter of smog has sparked an Internet outcry and repeated calls from state media for action.

Public anger continued to grow as the now familiar sight of mask-wearing pedestrians venturing out into heavily-polluted streets was broadcast regularly on television.

"I have lived in Beijing for four years and I have not seen it this bad before," said domestic cleaner Jiang Hua, who is originally from the central province of Henan. "It just seems so prolonged."

Pedestrians also vented fury at having to wear protective face masks.

"Wearing a face mask is annoying. As soon as I want to take a photo, I have to take the mask off because my glasses fog up," said Liu Lili, a tourist from the southern province of Guangdong.

Visibility in central Beijing was reduced to 300 metres, according to CCTV, causing 36 flights to be cancelled.

The broadcaster also showed images of the eastern province of Jiangsu -- some 1,000 kilometres (600 miles) south of the capital -- covered in a thick blanket of smog.

The US embassy's air quality index reading for Beijing stood at 301 and "hazardous" at 5:00 pm on Wednesday. The index rates a reading over 150 as "unhealthy" and above 300 as "hazardous".

The Beijing Municipal Environmental Monitoring Centre gave the figure as 286 at 4:00 pm, indicating the capital's air was "heavily polluted".

Both had stood at well over 400 on Tuesday, with the US embassy statistic peaking at 517.

China's pollution problems are blamed on rapid urbanisation and dramatic economic development.