The organizers of the Beijing Olympics on Friday announced a series of changes to the competition schedule in a bid to ensure all events finish ahead of the closing ceremony on Aug 24.

They affect the volleyball, softball, gymnastics and canoeing/kayaking competitions, each of which has had its starting time brought forward or put back, Liu Wenbin, deputy director of BOCOG's sports department, said at a press conference at the Olympic Main Press Center.

"Further changes to the schedules are possible, as not all of the National Olympic Committees (NOC) have confirmed their lineups with us," he said.

"The NOCs have given us their long lists of athletes but some have yet to provide us with their final squads. We will continue to adapt the schedules until we know the exact number of competitors in each event."

The final event of the Games will be the men's marathon, which must finish before the closing ceremony, Liu said.

"In the event of bad weather on the final day of competition, a contingency plan will come into force to ensure it finishes before the ceremony," he said, without elaborating.

The men's marathon is scheduled to start at 7:30 am on Aug 24, starting from Tian'anmen Square and ending at the National Stadium.

Any changes that are made to the event schedule will be in line with the rules and decision-making procedures of the affected sport's International Federation, Liu said.

"The timing of some events might be altered in the event of very high temperatures, for example," he said.

On the subject of pollution, Liu said the air quality in Beijing is up to standard.

"Two days ago, Gilbert Felli and Dr Patrick Schamasch from the International Olympic Committee said there was only a low chance that events will need to be rescheduled because of poor air quality, and that it will not be detrimental to the health of the athletes," he said.