BEIJING — Beijing will pull half its 3.3 million vehicles off the roads during the Olympics, betting the move, plus a stringent ban on construction and heavy industry, will clean the city's noxious air when the games open in seven weeks.

Under the temporary plan announced Friday, vehicles will be allowed on the roads on alternate days — according to even or odd car registration numbers — from July 20 until Sept. 20.

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

A deadly earthquake last month and fierce human-rights protests on international legs of the Olympic torch relay have removed some of the scrutiny from Beijing's chronic air pollution. But sparkling venues and $40 billion spent to improve infrastructure have not disguised the fact that air quality remains a contentious issue for the games.

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

"Ironically, the one place where expectations are so low are on the environment, where China may come out looking better than people thought," Victor Cha, director of Asian Studies at Georgetown University, said in an e-mail.

"Pea-soup air at the opening ceremony would be their worst nightmare, however," Cha added.

The Beijing Olympics take place on Aug. 8-24 followed by the Paralympics Sept. 6-17. About 500,000 foreigners are expected for the games with 10,000 athletes and about 30,000 journalists set to attend.

Beijing has long said it would limit the number of vehicles on roads for the Olympics, but Friday's plan was the most detail given, including the specific dates when the rules would take effect.

Under the traffic plan, between 30 and 70 percent of 300,000 government-operated vehicles will be taken off the roads. Officials also expect a strong increase in the use of public transportation, with several new subway lines set to open. Several others have opened in the last year.

The traffic plan was announced on a day when Beijing sweltered under a thick haze of pollution, limiting visibility to a half mile. Conditions were even worse on Thursday, although both days the Beijing Environmental Protection Bureau's Web site described conditions as only "slightly polluted."

"Perception is often different from the scientific monitoring statistics," said Du Shaozhong, deputy chief of Beijing Environmental Protection Bureau. "We base our findings on data."

"We now have 27 monitoring stations which all use state of the art equipment," Du added.

Du had repeatedly denied charges that officials moved the monitors away from polluted areas to get better readings.

"We have the confidence and capacity to provide good air quality for the Beijing Olympics," Du said. He estimated car emissions would be cut 63 percent by the ban and other measures.

The plan will also prohibit most vehicles entering the city from outside Beijing. These vehicles will need special permits and will need to meet air quality standards. Officials said violators would be punished under "applicable regulations," but gave no specific details about fines or other sanctions.

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

A similar plan was tested last summer in a four-day period, and the measures announced Friday were widely expected.

Five days after the July 20 ban goes into effect, special Olympic traffic lanes will be begin operating and will stay in place until Sept. 25. The city will set aside 165 miles of roadway, where certified Olympic vehicles will be allowed to move from hotels, Olympic venues and the Athletes Village. The average speed is expected to be 35 mph.

Setting aside special lanes is the only way to move traffic through Beijing's chaotic, crowded road system.

Officials announced several months ago that during the same two-month period, most construction in the city will halt, heavy industries will close and even spray painting will stop to clean Beijing's air. Some reports suggested that production cutbacks will come even sooner.