Greece confounded critics of its Olympic preparations on Friday when the futuristic roof over the main Games stadium took shape with the delicate meeting of two giant arches to support the translucent blue structure.

Three weeks after the first arch was pushed into place, the second 78-metre high white steel structure slid along tracks to meet the first arch to celebrations of construction workers.

"The sliding is done. It is finished," engineer Costas Mathiopoulos told Reuters. "It happened in about 24 hours."

With the arches in position -- the greatest architectural and engineering feat of the Games -- workers have now just to install thousands of blue carbon plastic-like panels that will offer shading against soaring summer temperatures.

It also opens the way for urgent landscaping work around the stadium that still resembles a huge construction site 70 days before the August 13-29 Games.

The roof had been the International Olympic Committee's biggest worry about Games preparations.

Organisers see the roof, designed by award-winning Spanish architect Santiago Calatrava, as a Games's architectural legacy that will light up Athens skyline for the future.

Many architects regarded the roof as an engineering challenge bordering on the impossible because of weights involved.

The two arches span more than 300 metres each, are nearly as tall as the Sydney Harbour Bridge and spread across the stadium like a giant blue spider's web.

They will carry 18,000 tonnes of panels and lights as well as broadcasting and security cameras.