They are holding in reserve further plans to reduce the number of cars on the roads and shut more factories, if projections show unacceptable conditions in coming days.

The pollution index for Tuesday was between 90 and 110, moderating from 95 to 115 overnight. China regards an index level of less than 100 to be a "blue sky day".

Many athletes pouring into Beijing ahead of the Games appeared to be more struck by the heat than the pollution.

"The humidity's quite fun, it's like rowing through a steam room," said British rower Olivia Whitlam.

Renata Ribeiro, a Brazilian beach volleyball player from Rio de Janeiro, thought fears about pollution were overstated.

"We'd been told it was absolutely terrible so we were prepared for much worse. It's actually not that heavy today. We're breathing fine," she said.

However, for athletes of endurance events the smog could pose a major problem and the International Olympic Committee (IOC) has said it might reschedule events if the pollution was too bad.

The local Games organisers, BOCOG, have pledged to finish the Games on Aug. 24 so most observers believe the IOC would switch the men's marathon, due to take place on the final day, to another Chinese city if the air quality was deemed too poor.

The IOC has not said what it regards as an acceptable pollution level for the marathon.

Beijing would be cloudy and hot through Thursday, the China Meteorological Administration said.

A sparkling weekend in Beijing had brightened hopes that the anti-pollution measures were working but Monday was muggy and smoggy. (Additional reporting by Jane Barrett and Crispian Balmer) (Editing by Jon Bramley and Robert Woodward) (For more stories visit our multimedia website "Road to Beijing" at http://www.reuters.com/news/sports/2008olympics; and see our blog at http://blogs.reuters.com/china)