noun
large crowd; huge audience
Refers to a very large gathering of people watching an event, such as a sports match, concert, or public spectacle. Emphasizes the scale of the audience.
スタジアムは大観衆で埋め尽くされた。
The stadium was filled with a huge crowd.
彼は大観衆の前でスピーチをした。
He gave a speech in front of a large audience.
大勢 means 'a large number of people' in general, not specifically spectators. 大観衆 is limited to crowds watching an event.
Compound of 大 (dai, 'large') + 観衆 (kanshū, 'spectators'). The reading is a straightforward combination of the on'yomi readings of the kanji.