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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The high of 35C was recorded on a Bakerloo line train between Piccadilly Circus and Oxford Circus at 2pm.
Weather forecasters say temperatures could reach 35C, beating the previous record of 34.6C in the scorching summer of 1976.
After years of political manoeuvring over the future expansion of Heathrow Airport, there has been an important development this lunchtime.