Gold Challenge invites people of all ages to test themselves in Olympic and Paralympic sports and raise £20m for charity by the end of 2012.
Those who raise the most, a number of schools and people selected by ballot will take part in 100m or 4X100m races.
Organisers hope 100,000 people will enrol to help their chosen charity.
Former Olympic gold medallist Sally Gunnell said Gold Challenge was an enormous opportunity for the public to get involved in the Games, while getting active.
"What a great opportunity to get inside that stadium before any of the athletes," she told BBC London at the event's launch in Stratford's Olympic Park.'Legacy project'
The 1 April event will include a parade for several thousand children and adults as well as a number of celebrity and athlete races. Tickets will go on sale early next year.
It will be hosted by the London Organising Committee for the Olympic Games and Paralympic Games (Locog) as a key test event ahead of London 2012.Would-be participants can register at the Gold Challenge website, which gives training advice and key information guides on the different sports and charities involved.
Individuals or teams will be able to participate in up to 30 types of sport while fundraising for any one of London 2012's 150 partner charities.
To complete each sports challenge, people must participate in at least three hours of coached activity.
Locog Chairman Lord Coe said the event would be a "fitting way to say thank you to those who have been inspired by the Games to get active and also raise money for charity".
Hugh Robertson, Minister for Sport and the Olympics said: "What greater incentive for people to get involved than an opportunity to run on the track in the iconic Olympic Stadium for charity."
Gold Challenge is one of the official London 2012 "legacy projects", and aims to harness the excitement around London 2012 to inspire people of every age and physical ability to play sport, while raising money for charity.
Doubts have previously been expressed over whether the promised sporting legacy for the UK would be achieved from hosting the Olympics.
In September, British Olympic Association Chairman Lord Moynihan warned that the country was "a long way from delivering a step change in sport".