Photo taken on March 14, 2011, shows the countdown clock marking 500 days to the 2012 London Olympics on the Trafalgar Square in central London, Britain.(Xinhua/Zeng Yi)

The countdown clock was unveiled in Trafalgar Square in London as the 500-day countdown to the start of the London 2012 Olympic Games began on Monday.

The clock was revealed by four Olympic gold medallists from Britain and will be counting down to the evening of the 27 July 2012 in days, hours, minutes and seconds.

LOCOG Chair Sebastian Coe said: "The launch of the OMEGA countdown clock is an important milestone for any Olympic Games and is something of a tradition within the Olympic Movement. It will be a daily and hourly reminder to everyone who visits Trafalgar Square that the countdown to the start of London 2012 has well and truly begun and that the greatest show on earth is soon coming to our country."

The clock, made from steel, is 6.5 metres high, 5 metres long and weighs around 4 tonnes.