BEIJING, Aug. 17 (AP) -- City officials yanked hundreds of thousands of private cars off Beijing's streets Friday to test whether a car ban could clear health-threatening smog and ease gridlock during next year's Olympic Games.

The test is a challenge for Beijing Olympic organizers and city authorities to see if they can balance the need to dampen pollution without angering a growing middle class that has caused an explosion in private car ownership over the past five years.

"We just got used to driving and suddenly we're not allowed?" said Zhang Jie, a stock trader who bought her first car _ a Chinese-made Chery QQ subcompactlast year.

"It's not good to forbid so many people from driving," said Zhang who planned to take a taxi to work. "I feel like the government should be able to come up with a better plan than this."

The four-day ban uses an odd-even license plate number system, so only vehicles with license plates ending in odd numbers were allowed on the roads Friday, the 17th of the month. The official Xinhua News Agency said an estimated 400,000 private cars were affected Friday _ more than a tenth of the city's 3 million vehicles.

Subway cars were packed with commuters during the morning rush hour. Traffic on the main roads flowed at a brisker pace than normal, while police maintained a normally visible presence.

Traffic and air pollution have emerged as key logistical problems for the city as it gears up for the games. Beijing has been adding cars at the rate of about 1,000 new vehicles a day, a pace that is not expected to slacken over the next year. International Olympic Committee president Jacques Rogge warned last week that some Olympic competitions might be postponed if the city did not clean up the pollution.

To get drivers to heed the ban, the city was using a combination of penalties and persuasion. Violators were to be fined 100 yuan (US$13; £á10) if caught driving in the city and ordered to return home, a Beijing Traffic Police spokesman, who would only give his surname, Yan, said Thursday.

State-controlled media, including Beijing's popular traffic radio station, broadcast appeals all week, reminding drivers that by obeying they were participating in the Olympics.

Zhu Xiaobin, chairman of the 4,000-member Zhu Shanren car club, said his members understood the need for the restrictions and would follow them. But, he added, the government should consider using carrots instead of sticks to get people off the roads in future. He suggested that people who abstained from driving during the games should be given a reduction of the road maintenance fees all car owners are required to pay.

Despite some grumbling, drivers mostly complied with the ban on Friday.

"It's for the Olympics so we've got to do it," said Li Hui, a consultant and car owner who added he would take a taxi to work. "This is a big thing for our country. Everyone should support it and put the greater good ahead of their own personal interests."

Other than taking taxis, many commuters opted to car pool, take buses or the subway or take the day off work. During the test period, the city had more buses running for longer hours. The city's subway lines were also keeping slightly longer hours.

Jin Guizeng, a retired gas company technician, said he planned to park his odd-plated car Friday and Sunday and watch some television. He said a few Beijingers would probably try to flout the rule by taking their license plates off or swapping their plates.

"You see it all the time, nobody does a thing," said Jin.

Traffic police rarely pull cars over in Beijing. Instead speeding and other moving violations are often caught by the capital's vast network of traffic cameras and fines sent by mail.

Emergency vehicles, taxis, buses and other public-service vehicles are exempt as were those given special passes by Beijing Olympic organizers to attend sports test-events around the city.

Beijing's often surly cabbies were cheered by the glut of customers.

"The traffic ban is good for us, it brings better business," said a driver who would only give his surname, Chang.

Details of the plan were announced last week by city authorities, who said the four-day plan would take 1.3 million cars off the road. It was not clear how the total figure was calculated and Olympic organizers and Beijing police said they could not immediately clarify.