Justin Bieber -- Accused of ANIMAL CRUELTY ... by Hamster Org.

Justin Bieber issued a DEATH SENTENCE to his pet hamster PAC when he gave it away to a screaming fan (who promised to love and cherish it forever) outside of a concert earlier this month ... this according to the California Hamster Association (yes, there's actually a California Hamster Association).

TMZ spoke to a rep for the group ... who tells us Bieber committed an act of "animal cruelty" when he gifted away the animal ... even if his intentions were good.

The CHA tells us ... hamsters are fragile creatures that "often succumb quickly to illness and death," especially when faced with "sudden environmental change."

In short ... "The moment that hamster was handed off to a screaming girl in a harsh, frenzied environment was likely the moment it gazed at the short path to its doom."

DOOM!!!!!!

So what should Bieber have done instead? The CHA says if Justin no longer wanted the pet, he could have left it in the care of an animal shelter or rescue center ... so they could've handed it off to a responsible new owner in a safer manner.

As for PAC, he's currently alive and well in the care of Bieber's 18-year-old super-fan Tori ... who has vowed to treat the animal like a tiny rodent god.

So far, no comment for Bieber.