Workers and officials have celebrated in Greece after efforts to start raising a grand arch over the main Olympic stadium went without a hitch.
International Olympic Committee (IOC) officials watched as one half of the hi-tech roof began to slide into place.
So far the roof has moved about two metres (6ft) and it is expected to be in place by the end of the week.
Delays have caused fears that some Olympic facilities will not be ready for the opening of the Games in August.
The minister overseeing the building work for the Olympics, Fanni Petralia, smashed a bottle of wine on the foot of the arch of the roof as it began to slide towards the stadium.
The BBC's Richard Galpin at the stadium says it was a critical moment.
He adds that the roof is a spectacular and pioneering piece of architecture and while much could have gone wrong, it all went smoothly.
Mrs Petralia said their gamble had paid off.
"Today we won the first bet and now we are going to prove the next step which is the big bet, that Greece can organise a unique and safe Olympic Games.
The other half of the roof is due to move later this month.
One aim of getting the arch moving on Monday was to show the IOC that Athens' Olympic preparations were back on track, correspondents said.
Huge anticipation awaited the installation of the 18,000-tonne roof, whose ambitious design - by Spanish architect Santiago Calatrava - has been hailed as a marvel of engineering.
The roof was expected be fully in place by the end of April, but delays have held up work on the rest of the stadium.
Olympic officials had said the roof needed to be finished by 20 May, or contingency plans would have to be adopted to protect spectators and athletes from the soaring Athens temperatures in August.
But now the committee is simply saying the work must be finished by the end of June to allow sufficient time for work on the rest of the stadium, as well as for test events and rehearsals.
IOC officials were spending the day touring Olympic venues to check progress made since their last visit.
Building work has been plagued by delays and about a third of venues are still unfinished.
Officially the IOC says it is confident the Athens games will be a success, but there is a massive amount of work still to be done to get everything ready before the opening ceremony on 13 August.