The Mayor's Senior Advisor for Volunteering, Charities and Sponsorship, Veronica Wadley, said:
"2012 has been an extraordinary year for London and the achievement of hosting the Games has shown what a terrific welcome we can give visitors. With the help of the Team London Ambassadors and other volunteering programmes, we want to build on the city's reputation for being able to deliver the most fantastic place in the world to visit."
More top news
Tree surgeon Ben Atkinson could be seen balancing on beams half way up the tower.
The flag was seen flying from the scaffolding around Big Ben.
Campaigners calling for a second referendum on Brexit are set to march through central London at midday.