If the circumstances are right (other people around or with you) and you feel up to it, return litter to its owner and ask that they dispose of it properly. Have exactly what you want to say in mind and say something like "Sorry, excuse me, I think you've dropped something" followed by, "there's a bin over there." to an offender.
Avoid being confrontational and be prepared for the tirade of abuse that occasionally follows. Also have answers to the common retorts to hand.
Excuse me, I think you left something behind.
Sorry, is this yours, you just dropped it.
I do hope that you'll be taking your litter with you when you leave.
Common defiant retorts:
I'll do what I like.
If I didn't do this I'd be doing someone out of a job!
I'm sure street cleaners would rather have a job doing someting else/the money was spent some other way.
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