Beijing's Olympic shutdown begins Sunday, a drastic plan to lift the Chinese capital's gray shroud of pollution just three weeks ahead of the games.

Half of Beijing's 3.3 million vehicles will be pulled off the roads and many polluting factories will be shuttered. Chemical plants, power stations and foundries left open have to cut emissions by 30 percent - and dust-spewing construction in the capital will be halted.

In a highly stage-managed Olympics aimed at showing off the rising power of the 21st century, no challenge is greater than producing crystalline air for 10,500 of the world's greatest athletes.

"Pea-soup air at the opening ceremony would be their worst nightmare," said Victor Cha, director of Asian Studies at Georgetown University.

Striking venues and $40 billion spent to improve infrastructure cannot mask Beijing's dirty air. A World Bank study found China is home to 16 of the 20 worst cities for air quality. Three-quarters of the water flowing through urban areas is unsuitable for drinking or fishing.

International Olympic Committee president Jacques Rogge has repeatedly warned that outdoor endurance events lasting more than an hour will be postponed if the air quality is poor.

Under the two-month plan, vehicles will be allowed on the roads every other day depending on even-odd registration numbers. In addition, 300,000 heavy polluting vehicles - aging industrial trucks, many of which operate only at night - were banned beginning July 1.

Five days after Sunday's traffic ban goes into effect, special Olympic traffic lanes will begin operating until Sept. 25, a plan that has been used in previous games. Beijing is setting aside 165 miles of roadway on which certified Olympic vehicles will be allowed to move from hotels, Olympic venues and Athletes Village.

To further ease congestion, employers are being asked to stagger work schedules. Public institutions will open an hour later than normal and two new subway lines scheduled to open Sunday should also bring relief.

The plan to clean the gray air seems to match the high-security tone of the games, which will be policed by 100,000 officials.

Razor-wire barriers and soldiers standing at attention guard the outskirts of the Olympic Green area and the Chinese have even installed ground-to-air missiles near one Olympic venue to protect it from possible attacks.

Security, tight visa rules and inflated hotel prices seem to be keeping foreigners away. Many nightspots near Olympic venue are being closed by security officials, who say the games are under threat from Muslim extremists in China's western Xinjiang region.

Beijing organizers are also in a protracted showdown with TV broadcasters, who are seeking free movement and reporting during the games. China's communist government seems to fear being embarrassed during the games by pro-Tibet activists, local dissidents or critics of China's human rights policies.

The gigantic experiment to curb pollution could still go wrong.

Veerabhadran Ramanathan, an atmospheric scientist at the Scripps Institution of Oceanography at the University of California San Diego, said unpredictable winds could blow pollution into Beijing despite factory shutdowns in the city and five surrounding provinces.

Ramanathan is leading a multinational research project in tracking Beijing's pollution before, during and after the Olympics.

"Reducing the local emissions is going to reduce the local pollution, but is that sufficient to help the athletes breath cleaner air? This is going to depend on the winds," he said.