The People's Poppy, a 20ft square floor display, features photographs of 3,500 people helped by the charity.
It is only in London for a day, before going on display in Sheffield, Cardiff, Liverpool, Cornwall and Manchester.
The legion is launching a new campaign to reach those who are entitled to help, but do not ask for it.
"We have created the Poppy Support brand to help us reach the many thousands who need advice and hope in times of despair "
Artist Helen Marshall used individual photographs to create the People's Poppy, which the legion hopes will encourage more people to ask for help.
It will be used to develop services in advice, funds, homes, volunteers, holidays and travel.
Legion head Ian Townsend said: "To most, the poppy is a symbol of reflection and remembrance.
"We have created the Poppy Support brand to help us reach the many thousands who need advice and hope in times of despair and, above all, to offer support when they need it."
He said one in six people in the UK are entitled to help, but many are unaware or feel too proud to ask.
"We're encouraging everyone to look behind the poppy image to think of someone who could benefit from Poppy Support."
Ms Marshall said there were pictures of people from all backgrounds, from school children to veterans.
"The age range displayed in the photographs is enormous and is a vivid reminder of how wide the legion's net can be cast," she said.
"Working on the project has been a very satisfying and moving experience."
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