BEIJING (AP) -- The Beijing Olympics are just a year away.

Sprawling Tiananmen Square was the center of celebrations Wednesday as countdown clocks across Beijing and other host cities reached the one-year mark at 8:00 p.m. (1200 GMT), exactly 12 months before the Games' opening ceremony.

Dancers cavorted, singers sang and politicians spoke of the smooth progress toward preparing for an event the communist government hopes will showcase the country's rising political and economic clout.

"From what we have seen so far, the preparations for Beijing 2008 are truly impressive in every regard," International Olympic Committee president Jacques Rogge said. "I don't think we have ever seen preparations on this scale."

Beijing's new anthem -- the just-released pop song "We're Ready" -- opened the ceremony, sung by a chorus of Chinese celebrities on a stage surrounded by banks of searchlights.

China's Communist government has been efficient in building venues. Except for the iconic "Bird's Nest" National Stadium, all of the 37 venues are to be finished by the end of this year. Venue construction has eaten up only a part of the US$40 billion (euro29 billion) being spent on new subway lines and skyscrapers to remake the capital.

There have been few delays, and the US$2.1 billion (euro1.52 billion) operating budget has been offset by the vast revenue expected from TV and sponsorships. That has allowed attention to focus on Beijing's choking pollution, campaigns to "civilize" the city and the risks involved for China's authoritarian government.

Normally cautious with his words, Rogge warned hours before Wednesday's big party that some events next year could be postponed if the air is too dirty.

"Yes, this is an option," Rogge told CNN. "It would not be necessary for all sports, sports with short durations would not be a problem. But definitely the endurance sports like the cycling race where you have to compete for six hours, these are examples of competitions that might be postponed or delayed to another day."

Despite billions spent to move refineries and steel mills out of town, Beijing has been blanketed for weeks by choking industrial smog.

To guarantee clean air during the 17-day Olympics, about a million of the city's 3.3 million vehicles are expected to be kept off the roads. Officials are also hoping to control the weather. Meteorologists began tests last month, firing rockets to disperse rain clouds -- a move to guarantee sunshine. They've also fired rockets containing sticks of silver iodide to induce rain to clean the air.

"They've told us the factories will be closed for three months in 2008 and that they will have a directive to encourage residents to stay off the roads with their cars," said Steven Roush, chief of sport performance for the U.S. Olympic Committee.

Like other national Olympics bodies, the USOC is monitoring the quality of Beijing's air, laden with ozone, dust and exhaust from some aging vehicles.

Image is important with 550,000 foreign visitors and about 22,000 accredited media set to attend. In addition, up to 10,000 non-accredited journalists are expected.

Old habits like spitting in public, jumping ahead in line and littering are under siege in various campaigns aimed at improving the behavior of China's citizens. Everyone -- from taxi drivers to Olympic volunteers -- is being pressured to learn some English.

Revenue from local sponsorship is expected to be at least double that of Sydney or Athens, reported to reach US$1.5 billion (euro1.09 billion), with billions more spent on advertising and promotional campaigns.

Although many athletes will eat specialized diets provided by their own teams, Olympic organizers also have promised to track food electronically from the field to the consumer. The state-run China Daily newspaper reported recently that mice will be used to test food samples.

The biggest security threat -- to the Chinese government -- may come not from al-Qaida but from protesters hoping to highlight causes like labor rights or China's role in the Darfur crisis. Other protests may center on Tibetans who seek autonomy, religious activists, and calls for media freedom and the release of political prisoners.

"Great achievement is always accompanied by great challenges," said Jiang Xiaoyu, an organizing committee executive vice president. "While the Beijing Olympics are a great opportunity, we are also confronted with huge challenges."