A new clock has been unveiled in London's Trafalgar Square to mark the 500-day countdown until the start of the 2012 Olympic Games.

Omega, official timekeeper of the London Games, and the London Organising Committee of the Olympic Games and Paralympic Games (LOCOG) launched the clock. 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." The unveiling of the Countdown Clock comes as 6.6 million tickets for Olympic events went on general sale to UK and European Union residents.

A 42-day application process will enable people to apply for tickets, which are not being sold on a first-come, first-served basis. The process closes on 26 April.