Fireworks surround a clock counting down the days to the start of the 2012 Olympic Games, launched to mark the 500 remaining days during a ceremony in Trafalgar Square, central London, March 14, 2014. [Photo/Agencies]

LONDON - The countdown to the London 2012 Olympics took physical shape on Monday with the unveiling of a clock in the central Trafalgar Square ticking away the 500 days to go to the opening ceremony.

Four British Olympic gold medallists - rowers Pete Reed and Andy Hodge and sailors Iain Percy and Andrew Simpson - hauled on a thick rope to pull screens away from the steel clock near Nelson's column.

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Measuring 6.5 metres tall and five metres long, the digital display will count down the days, hours and minutes in the same square where Londoners gathered in 2005 to hear their city announced as winning bidders.

"For many thousands of Londoners and people around the country, this was the moment where I suppose the countdown really started," London organising committee chairman Sebastian Coe told Reuters.

"This is probably one of the most iconic landmarks in London, it draws millions and millions of people from around the globe every year and we thought this was a pretty obvious place to put it.

"The clock draws very heavily on the geometric lines that we've brought the logo into the world with, but also the fact that Greenwich Mean Time is that line that the world sets its clocks to."

With tickets going on sale from midnight GMT, the next big milestone for organisers will be the one year to go point on July 27.

"This clock reminds us that we've still got quite a lot to do," London mayor Boris Johnson told the crowd gathered in the evening cold.