Lots of people

crowd, throng
群衆

Example:
A crowd gathered around the apparently-famous runner.
あのどうやら有名らしいランナーの周りに群衆が集まってきた。

Comment:
Throng is a less-used word, but it means the exact same thing.

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packed
混んでる

Example:
The streets were packed with people watching the fireworks show.
道は花火大会を見てる人で混んでた。

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jam-packed
すし詰めの

Example:
The trains were jam-packed on the way home. It sucked.
帰り電車はすし詰めてた。最悪だった。