London is marking the two-year countdown to the 2012 Olympics with a series of activities across the city.

Organisers issued a call for 78,000 Olympic volunteers, threw open venues for star athletes and opened the first official merchandise store.

Tuesday is exactly two years to go until the opening ceremony of the Games on July 27, 2012.

London mayor Boris Johnson appealed for local volunteers to help visitors get around the city during the Games.

He said they would be the “polite, courteous, smiling - without being irritating - faces of the city.”

Johnson said London will host “the greatest party on earth.”

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