Tom Sizemore -- Proof My Arrest Was a Mistake!

Tom Sizemore is adamant ... he never should have been arrested at his L.A. apartment last week -- and TMZ has obtained the actor's proof ... a document stating he completed his required 80 hours of community service.

As we previously reported, cops arrested Tom on an outstanding battery warrant -- for allegedly failing to prove he completed his community service stemming from his 2009 spousal battery case.

But sources close to Sizemore tell us ... the actor insists the warrant was issued in error ... because he has a letter from the Hollenbeck Police Business Council proving he fulfilled his community service obligation.

Sources close to Tom tell TMZ, the actor's old attorney was supposed to file important documents with the court -- showing proof of completion of Tom's community service -- but never did ... so the court issued the warrant.