In front of Bird's Nest, the National Stadium, fully-armed workers launched a new war on Friday against the capital's rats, flies, mosquitoes and cockroaches.

Such pests were dubbed the "Four Harms" in 1950s by late Chairman Mao Zedong, and later turned into a long-term national campaign to enhance public awareness of disease prevention.

The disparity between the strength of the two sides was apparent as a spray machine of the latest technology blasted a piece of grassland with tiny fog tablets, two or three of which could trap mosquitoes and flies immediately after contact.

Liu Zejun, Beijing Patriotic Sanitation Movement Committee office director, said the machine could spray the pest-killing fog over an area of 12 square km each hour.

"The spray uses water as a solvent which does no harm to plants. The killing effect can last three days to a week."

On Friday, the country launched a new drive aiming to eradicate the larva of vermin pests prior to the their appearance en masse. Sun Xianli, the committee director, said the activity was an important one before the August Games.

The exercise covered the whole city, especially Olympic venues, training facilities and areas surrounding them within a 2,000-meter radius.

Zeng Xiaofan, a Beijing Center for Disease Control and Prevention official, said mosquitoes would get no closer than two km to the areas adjacent to the Olympic venues and fields after the mass killing.