Britney Spears got her bump and grind on at a Bally health club yesterday - but not before tossing out a room full of aerobicizers.

We're told participants in the regularly scheduled exercise class were majorly PO'd that they got the boot. Gym staff tried to console the electrolyte-heavy crew with free t-shirts and water bottles - some of which literally were thrown back at them.

TMZ spoke with Bally's Chief Marketing Officer John Wildman who admitted that booting regulars for Spears was their fault -- someone at the gym double booked the room. He tells us "it will never happen again," and says members were offered access to other programs the gym offered.

Feel the burn!

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