From Wandsworth to Camden, hundreds of people have set to work in their local communities to mark Make A Difference Day.
Many have been inspired by the example set by London's 70-thousand Olympic Gamesmakers.
Piers Hopkirk has the story.
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A lamp in the tower which houses Big Ben is to be switched off for the first time in more than 70 years.
Sir Bruce entertained audiences for more than seven decades.
Work in the tower will also pause over the bank holiday weekend.