Half of the Chinese capital'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 per cent - and dust-spewing construction in Beijing will be halted.

The measures are the final lap of China's long-running efforts to ensure clean air for 10,500 of the world's greatest athletes.

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.

With Beijing's reputation for dirty air that sometimes reduces visibility to 2km or less, some athletes have decided to delay their arrival until just before the Olympic opening ceremony on Aug8.

The authorities are hoping that the shutdown plan will give athletes clean air for at least the duration of the Games.

Under the two-month plan, vehicles will be allowed on the roads every other day, depending on whether their licence plates are odd- or even-numbered.

Five days after today's traffic ban goes into effect, special Olympic traffic lanes will be begin operating until Sept 25.

This plan has been used in previous games. Beijing is setting aside 265km of roadway where certified Olympic vehicles will be allowed to move between hotels, Olympic venues and the Athletes Village.

Meanwhile, employers are being asked to stagger work schedules.

To further ease traffic gridlock and soften the impact of the vehicle ban, the authorities opened two new subway lines, delayed from a planned late-June start but just in time to carry passengers banned from using their cars.

They include a link to the main Olympic stadium and a high-speed line running from Beijing's new airport terminal to the city centre, adding an extra 58km to the over-stretched subway system.

Passenger service has yet to start, however, on a third line that will serve the Olympic Green and which was the site of an opening ceremony and test ride for the media yesterday.

The three new lines, which cost 22.3 billion yuan (S$4.4 billion), are part of massive infrastructure plans to ease transport during the Games.

Notwithstanding the shutdown, city officials have already claimed victory in the fight for clean air.

After nine years of strenuous clean-up efforts costing about 120billion yuan, Beijing now meets healthy air standards for 70per cent of the days in a year, compared with just 20per cent a decade ago, a top city environment official told reporters on Friday. But things could still go wrong.

Professor Veerabhadran Ramanathan, an atmospheric scientist at the Scripps Institution of Oceanography at the University of California at San Diego in the United States, said unpredictable winds could blow polluted air to Beijing despite the factory shutdowns.

Prof 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 breathe cleaner air? This is going to depend on the winds,' he said.

AP