The £250 million powerlines project is a key part of the wider work being undertaken by EDF Energy and National Grid to reinforce and upgrade the power network for east London and the London 2012 Games.
Olympic Delivery Authority chairman David Higgins said: "Removing the overhead pylons from the Olympic Park site is a symbol of the huge change the games is bringing to east London.
"The pylons will all be down by the end of the year, unlocking the area for the development of new homes, world-class sports venues and essential infrastructure."
The pylons are being replaced by two tunnels, each four miles long, to carry the power lines underground.
Digging the tunnels created 200,000 cubic metres of material - enough to fill Wembley Stadium - and the vast majority is being reused in construction of the Olympic Park.