Half of all cars belonging to the government and Communist Party were ordered off Beijing's clogged roads Monday, part of an effort to help clean the city's air ahead of the Olympics.

Despite the ban, the skyline remained shrouded in smog, and traffic was bumper-to-bumper during afternoon rush hour on freeways and Chang An Avenue, which cuts through the heart of the city.

Under the edict, announced on the city's Web site, half of all government and Communist Party cars were not being used from Monday until July 19.

On July 20, another rule begins that will ban half of Beijing's 3.3 million cars on alternate days, depending on whether their license plate numbers end in odd or even numbers.

In addition, 300,000 heavy polluting vehicles — aging industrial trucks, many of which operate only at night — will be banned from July 1.

The auto ban is part of an anti-pollution plan that also will halt construction and heavy industry during the Olympics.

"To meet the air quality standards and to realize safe and smooth traffic is our solemn promise to the international community," the Beijing government notice said.

Officials also expect an increase in the use of public transportation, with several new subway lines set to open. Several others have opened in the last year.

Officials said that 95 percent of the city's 66,000 taxis would be operating during the games, along with 21,000 buses.

Chronic gridlock and polluted skies have emerged as problems for the city as it gears up for the Aug. 8-24 Olympics, a source of tremendous excitement and national pride. About 500,000 foreigners are expected for the games, with 10,000 athletes and about 30,000 journalists.

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

Ground-level dust, soot and industrial emissions mixed with car exhaust creates a gray-brown haze that often blankets the city of 17 million.

"As a short term matter, restricting the use of the automobile is sensible and the only thing they could do right now," said Fred Salvucci, senior research associate at Massachusetts Institute of Technology's Center for Transportation and Logistics. "The big question is whether it will help enough."

Salvucci, who also served as Massachusetts' secretary of transportation for 12 years, said curtailing auto traffic should help by lowering congestion and thereby shortening the hours that vehicles travel.

"With less stop and go, and less congestion, vehicles travel less hours and engines will be more efficient and generate less pollution," he said.

In a dry run — and a sign that officials understand the gravity the problem — Beijing imposed similar traffic controls in August 2007 for four days, removing hundreds of thousands of private cars from city streets.

In 2006, about 800,000 private and bureaucratic vehicles were not allowed to take to the roads during a six-day meeting between Chinese and African leaders in another successful experiment.