。
The stadium was packed to overflowing with people.
Literal
Stadium [topic-は] overflow-[almost-んばかり] [gen-の] crowd [copula-だった].
~んばかり is a classical-ish grammar pattern meaning 'almost / just about (to) X,' attached to the ない-stem of a verb — 溢れん+ばかり 'almost overflowing.' It's still productive in literary prose: 泣かんばかり 'on the verge of tears,' 怒らんばかり 'almost angry.' The んばかり gives a vivid sense of something barely contained. 人込み 'crowd' is a compact compound (人 'person' + 込み 'packed').