We were buffeted by the running terrified crowd.
The article 'the' refers to the noun 'crowd'.
If you say 'the terrified crowd running' the article would make 'running' a noun, which is wrong. The interpretation that 'running' is used as a postmodifying attribute to 'crowd' is also incorrect because unlike the participles concerned, involved, etc. (the question concerned, the people involved) 'running' doesn't function that way.
Except in the set-phrase 'two days (/...) running.'
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