They also banned more than a million vehicles from its roads...
More than 6,500 traffic police were on duty across the city to ensure car owners observed the ban, while an extra two million more trips were expected to be taken on subways and buses during the day, officials said.
The four-day test is expected to be a prelude for a similar ban to be put in place for the duration of the August 2008 Olympics, as part of a range of measures to temporarily improve air quality for athletes and visitors.
Beijing is one of the world's most polluted cities, and poor air quality, blamed partly on the city's three million cars -- a number growing by 1,200 a day -- has long been a top concern for athletes and officials.
Those worries were exacerbated as a weeks-long smoggy haze that reduced visibility to just a few hundred metres (yards) on occasions marred the build-up to last week's August 8 one-year countdown to the Games.
International Olympic Committee president Jacques Rogge, in Beijing for the countdown, said then some events could be postponed if pollution was extremely bad, in what would be an unprecedented move to protect athletes' health.