Cops Taser and Cuff 7th Heaven Actor

Multiple witnesses tell TMZ ... Keegan -- who has starred in a bunch of movies and TV shows including "10 Things I Hate about You" [Joey Donner] and "7th Heaven" [Wilson West] -- was throwing a party in Marina Del Ray last night when cops rolled up and asked him to turn down the music. Witnesses say Keegan refused and became "very aggressive."

We're told officers slammed him to the ground and tasered him. One witness said they could hear Keegan screaming to let him go.

Keegan was eventually released and according to cops ... he was never placed under arrest.

A source who spoke to Keegan late last night told us "he was very shaken up" by the ordeal.