So many people jammed onto the Golden Gate Bridge for its 50th anniversary party that worried engineers did rapid calculations to make sure the span could support the weight, officials said today.

''The bridge flattened out - its whole arch disappeared,'' said Gary Giacomini, president of the bridge district board. ''The bridge had the greatest load factor in its 50-year life.

''Imagine the Golden Gate Bridge flattened out by the weight of human beings!''

The engineers determined that the bridge could support the weight, he said. #500,000 Packed Approaches Officials estimated that 250,000 people crowded the bridge deck Sunday morning to walk across the Golden Gate. More than 500,000 others packed the bridge approaches, but were denied access by the authorities.

Mr.Giacomini said officials regretted that so many were turned away, but added, ''In a way, I'm grateful because if they had gotten out there, maybe the bridge would have fallen down.''