UNITED NATIONS - Secretary-General Ban Ki-moon started a clock Wednesday that will count down the days, hours, minutes and seconds until the start of the Olympic games in London a year from now - on July 27, 2012.

The ceremony, attended by nine-time Olympic Gold-medalist Carl Lewis and three other US Olympians, coincided with a ceremony in London's Trafalgar Square where International Olympic Committee President Jacques Rogge invited sportsmen and women from around the world to compete in next year's Olympic and Paralympic games.

Ban powered up the clock, emblazoned with the London Olympics logo - the numbers "2012" in four jagged figures with the five Olympic rings - before a crowd of UN ambassadors and staff in his temporary office building on the north lawn of UN headquarters at exactly 2:30 pm EDT, the equivalent of 7:30 pm in London when the games will begin a year from now.

"We've all heard of Big Ben," Ban said, referring to the famed clock in the tower above London's Houses of Parliament.

"Somebody suggested another possibility. Since it will be placed in the United Nations, we might perhaps name it 'Big Ban,"' the secretary-general said with a smile as the crowd burst into laughter.