According to union officials, workers heard 'an enormous bang' and saw a 50m-long beam drop 1m, before being ordered to evacuate.
Although nobody was injured, it is understood all 3,000 workers were told to leave the stadium, designed by Norman Foster and HOK Sport, after the incident at 8.45am this morning. Most have now gone home.
The accident is the latest in a long line of disasters for development giant Multiplex.
The Australian company has already spent more than double the original £325 million budget on the delayed north London project, which is not expected to open until the summer - missing the deadline for this year's FA Cup Final.
At this stage, details of what went wrong remain sketchy, although the area of roof in question is believed to be the leading edge box at the front of the structure.
One source told AJ's sister publication Construction News: 'Once [the edge box] has been depropped it is supposed to stay where it is.
'There might be a bit of flex in the wind, but it's not supposed to drop by 1m. It's supposed to stay in place forever.'
Multiplex is now carrying out a full investigation into the incident. Neither HOK Sport nor Foster and Partners were available for comment.