The Olympic city was blanketed in smog that obliterated the sun today as pollution in Beijing reached dangerous levels.

The smog stretched more than 40 miles to the section of the Great Wall of China where the final leg of the Olympic torch relay was making its way along the fortifications.

The massive crowds lining the battlements were unable to see much more than a few hundred yards as Chinese athletes carried the torch on the wall, the key symbol of the country.

In Beijing, pollution was nine times the levels in London. The main Olympics venues were shrouded in the smog, which is at the centre of increasing concern for athletes.

Chinese officials had hoped clear skies over the past two days were a sign the smog was lifting, but today's levels were the worst of the week so far.

Pollution levels were recorded at 468 micrograms per cubic metre of air, compared to a reading in London's most polluted sites of 52 micrograms per cubic metre.

Olympic officials are examining the possibility of switching events if the smog is considered a danger to the athletes, particularly the two marathon events.

The number of cars on the streets of the Chinese capital has been cut and factories ordered to stop releasing smoke into the air.