Monitors at the United States Embassy in Beijing’s inner east said the concentration of airborne PM 2.5 particulates reached 845 micrograms per cubic meter late on Saturday afternoon, before falling below 700 after midnight.

These particles, 2.5 microns in diameter or smaller, are considered the most harmful to health because they can penetrate easily into human tissue.

The peak reading is roughly 35 times higher than guidelines issued by the World Health Organisation.

A reading above 100 is considered dangerous for sensitive groups while readings above 300 are classed as “emergency conditions” which are potentially “hazardous” for anyone who breathes it.

Residents could be seen grimacing after inhaling mouthfuls of air that they variously described as tasting like burning tires, industrial cleaning chemicals and concentrated car exhaust.

Others squinted into an airborne sludge, complaining of sore eyes, before hurrying indoors.

A Lancet study published last month said a record 3.2m people died worldwide from air pollution in 2010, four times the number a decade earlier, with 1.2 million of those deaths in East Asia.

This ranked pollution for the first time in the world's top 10 list of killer diseases, mostly because of vehicle exhaust.

The US Embassy pollution monitor is published on a Twitter feed, which is blocked in China, but which is picked up and placed on several popular Chinese websites and iPhone applications.

The service ran into political trouble in 2010 when the reading soared above 500, beyond the scale of the index at the time, and operators resorted to a new category: “crazy bad”.

The Chinese Government has complained about the US Embassy reading and told it to halt publication, with one diplomat calling it not only “confusing but also insulting,” according to a US diplomatic cable obtained by WikiLeaks.

Until recently the Chinese Government measured but refused to release PM 2.5 particulates, publishing only the larger and less dangerous PM 10.

Many online commentators and some analysts have questioned whether the official readings have been rigged, given the frequency with which readings have been recorded just within the level that it calls a “blue sky day”.

Some days have measured “beyond index” (or “crazy bad”) on the US index but “slightly polluted” on the official Chinese measure.

Beijing temporarily improved its air quality for the 2008 Olympics including by improving vehicle emissions standards, banning coal stoves and shifting heavy industry to poorer parts of the country.

Some of those gains have now been offset by an explosion in the numbers of cars on the roads.

The Beijing News last night said the new official 2.5 measure, which his based on the average of gauges at different points around the city, had reached 456. It also cited the peak US Embassy measure.

It said cold, moist atmospheric conditions could keep pollution at high levels until Tuesday.

The report advised children to stay indoors, adults to avoid exercise and all citizens to eat more pears and fungus soup.