Extreme air pollution in the Chinese capital Beijing has prompted authorities to increase the level of health warnings and order the temporary shut down of more than 100 factories.

Authorities urged the city's 20 million people on Wednesday to stay indoors and shut windows, eat a "balanced diet" and drink plenty of water. A third of all government vehicles have also been ordered off the road.

The director of Beijing's children's hospital said earlier this week that the "continuous smoggy weather has significantly increased the number of child patients".

One mother said she was worried that the air was worsening her son's allergic condition.

"The weather is just too terrible. And a respiratory infection actually has a pretty big effect on these kind of kids," she said.

Emergency measures were issued on news websites and microblogs, as well as in bulletins carried by the state broadcaster.

'Never this bad'

"I have lived in Beijing for four years and I have not seen it this bad before," said Jiang Hua, a domestic cleaner.

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

More than 43,000 people had voted in favour of new laws to tackle the smog in a survey posted by Pan Shiyi, a real estate tycoon and blogger running the campaign.

Earlier this month, the US embassy in Beijing recorded that the pollution in the city was 45 times above the recommended safety levels.

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