China's capital was shrouded in dangerous smog on Thursday, cutting visibility down to a few hundred metres as a count of small particulate pollution reached more than 25 times recommended levels.

A grey haze filled the sky, leaving an industrial, burning smell hanging in the air as PM2.5 reached over 500 micrograms per cubic metre in parts of Beijing, according to official statistics.

Figures from the US embassy, which also monitors air quality, reached more than 671 in the early morning. The WHO recommends an exposure level of no more than 25 over a 24-hour period.

The official air quality index reached the top of the scale at 500.

China's cities are often hit by heavy pollution, blamed on coal-burning by power stations and industry, as well as vehicle use, and it has become a major source of discontent with the ruling Communist Party.

"I can't believe how bad it is," said US tourist Richard Deutsch in Tiananmen Square in central Beijing, which was shrouded in heavy smog.

"I have never experienced air this bad," said Deutsch, who is from Vermont. "It's worse than LA. I feel kind of bad for the people. It's taking years off their life."

Authorities have become more open about pollution levels, in part as a response to public pressure, but officials have implied that it will take years before the situation improves.

The pollution has been linked to hundreds of thousands of premature deaths, and has tarnished the image of Chinese cities including Beijing, which saw a 10 percent drop in tourist visits during the first 11 months of 2013.

In Tiananmen Square, which is bounded on one side by the Forbidden City, a Unesco World Heritage Site, tourist guide Xiao Yan said: "There are less and less tourists coming to Beijing, it's normal as people keep on talking about the bad air quality on the internet."

China's State Council, or cabinet, said last year that "concentrations of fine particles" in the capital's air would fall by approximately 25 percent from 2012 levels by 2017.

Other major Chinese cities on China's affluent east coast, including Shanghai and Guangzhou, will see reductions of between 10 and 20 percent from 2012 levels over the same period, it said.

China is the world's biggest coal consumer and is forecast to account for more than half of global demand this year.

By mid-afternoon Thursday the air quality had improved, with the official PM2.5 figure down to 189 micrograms per cubic metre and the US embassy at 286.