March 15 - The official Olympic timekeeper Omega were today trying to fix the Countdown Clock after it stopped less than 24 hours after being unveiled in Trafalgar Square - a huge embarrassment for the company as London 2012 was the centre of attention with tickets going on sale to mark 500 days until the start of the Games.
Last night's launch had been attended by London 2012 chairman Sebastian Coe, London Mayor Boris Johnson, heptathlon world champion Jessica Ennis and Omega President Stephen Urquhart.
A statement from the company read: "We are obviously very disappointed that the clock has suffered this technical issue.
"The Omega London 2012 countdown clock was developed by our experts and fully tested ahead of the launch in Trafalgar Square.
"We are currently looking into why this happened and expect to have the clock functioning as normal as soon as possible."
The clock eventually restarted several hours later.
Earlier, Coe, American track and field legend Carl Lewis, five-time gymnastic gold medallist Nadia Comăneci, Britain's double Olympic swimming gold medallist Rebecca Adlington and 26 schoolchildren representing the 26 different sports were on hand at Tower Bridge to help promote the 500-day countdown and the start of the ticket sales.
In the UK and within the European Union, people can apply for the 6.6 million available tickets between today and April 26.
They can apply via www.tickets.london2012.com at any time during the 42-day application period.
As it is not a first come, first served system there is no advantage to applying earlier in the process.
"This is a momentous moment for all involved in London 2012," said Coe.
"This starts the journey for those who have been dreaming of getting hold of an Olympic ticket since the day we won the bid in 2005 - these really are the greatest tickets on earth.
"In the UK, people have 42 days to make their application and I urge everyone to take a look at what sports are available on what days and make their choice.
"If any sessions are oversubscribed we will run a ballot, which we think is the fairest way of allocating tickets."
Ticket prices, which were announced last year, start at £20 and range up to £750 for competition, while tickets for the Opening Ceremony start at £20.12 ($32.36) and go up to £2,012 ($3,236).
There are also schemes such as Pay Your Age for young people who are 16 and under while people aged 60 or over when the Games start can pay just £16 ($26) for some sessions.
"London will be a fantastic place to be in the summer of 2012 as you cannot beat the atmosphere of an Olympic Games," Lewis told insidethegames.
"Everyone who can needs to make sure they start signing up for tickets to see the best athletes in the world competing, and not just in track and field.
"Try out some sport you may not have seen, like handball and volleyball, and you won't regret it."
Postal applications will also be accepted in the UK, while on-site hospitality packages are available to purchase through the London 2012 Prestige Ticketing programme by clicking here.
Customers are advised to purchase London 2012 Olympic Games tickets only through official sources as it is illegal to sell Olympic tickets without the permission of London 2012.
To mark the start of the ticket sales, London has produced 13 different adverts, supporting eight sports, with each using athletes aspiring to be at the London Games in the venues that they will compete in if successful.
Among them are diving star Tom Daley and gymnastics world champion Beth Tweddle.
Daley said: "It's amazing and a great honour to be part of the London 2012 advertising campaign to promote tickets to the Games.
"All my family have been waiting for the day when they can hit the button to apply.
"It will be a once-in-a-lifetime experience for both athletes and spectators."
In addition London 2012 has also produced the "greatest tickets on earth" film with the help of the International Olympic Committee and the BBC.
The film, a history of magical moments from the Olympic Games, is designed to capture the magic of the Games and excite people that London is next.
To view the film, click here.
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March 2011: Countdown to London 2012 really on now with unveiling of Omega clock