Building work on a 115m-tall (377ft) spiralling tower in the Olympic Park, designed by artist Anish Kapoor, has begun.
The ArcelorMittal Orbit, costing £22m, will be located between the Olympic Stadium and and the Aquatics Centre.
Visitors will be able to climb on the structure to get panoramic views across the Stratford site in east London.
The artwork incorporating the five Olympic rings in a lattice of steel is expected to be built by March 2012.
London Mayor Boris Johnson, Lakshmi Mittal, chairman of the steel company ArcelorMittal, artist Anish Kapoor and Andrew Altman, chief executive of the Olympic Park Legacy Company, attended the ceremony.
Anish Kapoor said: "I am delighted that construction work has started.
"I would like to thank everyone who has shown their support for the project."
Mr Johnson said: "Recognised the world over, the Orbit will not only be an amazing piece of art, but it will draw visitors from every corner of the globe to East London."
The projected cost of the structure has gone up from £19.1m to £22.3m.
ArcelorMittal will provide £19.2m, with the remaining £3.1m being funded by London Development Agency.
The Turner Prize-winning artist will work with structural designer Cecil Balmond of engineering firm Arup for the project.