The clock was revealed by four Olympic gold medallists from Britain.
It will count down to the evening of July 27, 2012, Xinhua reported.
Sebastian Coe, chair of the London Organising Committee for the Olympic Games (LOCOG), said: "The launch of the 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 tall, five metres long and weighs around four tonnes.
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