The official countdown clock to the London 2012 Olympics has stopped just one day after it was unveiled in Trafalgar Square.

The clock was the centrepiece of celebrations to mark 500 days to go to the 2012 Games, which it counts down in days, hours, minutes and seconds.

But the digital display has now become stuck on 500 days and 7:06:56.

Officials from Omega say they are investigating the technical problems.