The deal between Westminster City Council and O2 makes the service available to all internet users, regardless of their current provider.
The network will initially cover Oxford Street, Regent Street, Trafalgar Square, Leicester Square, Piccadilly Circus and Parliament Square.
Other areas of Westminster will be covered in due course, with Covent Garden next.
The project has been paid for by O2 who hopes to use targeted advertising to recover the costs.
Whilst the scheme is being launched to coincide with the Olympic and Paralympic games the service will remain free of charge to all users after the games have ended.
Councillor Philippa Roe, leader of Westminster Council, said the free wi-fi was part of a “long term project to help make London one of the most technology-friendly cities in the world”.