One expert warned that drastic measures enacted to cut vehicle and factory emissions in the city were no guarantee that the skies would be clear during competitions.

The pollution was among the worst seen in Beijing in the past month, in spite of traffic restrictions enacted a week ago that removed half of the city's vehicles from roads.

Visibility was just half a mile in some places. During the opening ceremony of the Athletes' Village yesterday, the housing complex was invisible from the nearby main Olympic Green.

International Olympic Committee vice president Gunilla Lindberg, from Sweden who is staying in the Athletes' Village, said: "No, it doesn't really look so good, but yesterday was better. The day I arrived, Tuesday, was awful.

"We try to be hopeful. Hopefully we are lucky during the games as we were with Atlanta, Athens and Barcelona." The city's notoriously polluted air is one of the biggest question marks hanging over the games, which begin on August 8.

Temperatures of 32 degrees Celsius (90F), with 70 per cent humidity and low winds, created a soupy mix of harmful chemicals, particulate matter and water vapour.

The Beijing Municipal Environmental Protection Bureau said the air was "unhealthy for sensitive groups".

The Chinese leadership consider the Beijing Olympics a matter of national prestige, and efforts to clean up the environment were part of its meticulous preparations for an event it hopes will dazzle the world.

Athletes have been trickling into Beijing and were expected to begin arriving in large
numbers this week – though some were heading to South Korea, Japan and other places
to avoid Beijing's air for as long as possible.

Jacques Rogge, president of the IOC, has warned that outdoor endurance events will be postponed if the air quality is poor.