Air pollution levels in the Chinese capital Beijing are at hazardous levels for the third day running, with dense smog continuing to shroud the city amid freezing winter temperatures.

Citizens have been advised to stay indoors. The World Health Organisation warns that high levels of pollution are linked with respiratory infections, lung cancer and heart disease.

Many of those who ventured out wore face masks to protect against the thick layer of smog still covering the city.

Chinese dissident artist Ai Weiwei posted this picture of himself on Twitter, wearing a gasmask.

In Beijing and other northern cities, flights have been cancelled and traffic affected by significantly reduced visibility.

Air quality in China's cities is among the worst in the world, according to international monitors, made worse by the country's reliance on coal for energy and a rapidly-growing car market.

Until recently, a lack of official data made it difficult to monitor pollution levels in China, but this time TV and newspapers have clearly warned Beijing residents about the health dangers.