Here's Rihanna in a tiny bikini at a beach in Sopot, Poland this weekend ... where a giant pack of Poles lined up behind a fence to gawk.
Rihanna wasn't thrilled with the attention -- actually, she was pretty pissed ... tweeting, "Tried to have a beach day, that was a fail!!"
RiRi later griped that she felt "caged in like a f**king animal!"
Fun Fact: Sopot is a popular spa town for the rich and famous ... and sports the longest wooden pier in Europe.
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