Boris Johnson, the Mayor of London, said: “I urge everyone to enjoy the choirs, buy a rose and give generously on Alexandra Rose Day."
Events will start at the headquarters of Bank of America Merrill Lynch at midday.
The centenary collection will raise funds for three London charities: Downside Fisher, a youth club in Bermondsey; JusB, a youth club and counselling centre in Bromley; and EnfuseYouth, an innovative charity helping the young unemployed find jobs.
Downside Fisher will use the money to train young people to swim so they can apply to be deck hands at the nearby St Katherine Dock marina.
And funds raised for JusB will help pay for a youth outreach worker to offer support to young people in the area.