Rihanna

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.