Miley Cyrus Yells 'Free Britney' During Beale Street Festival

Miley Cyrus appears to believe Britney Spears was recently being held at a mental health facility against her will ... and the proof is in tweaked lyrics to her biggest song.

Miley was performing down in Memphis, TN Saturday at the Beale Street Music Festival, and while she was singing "Party in the U.S.A." ... the Britney part of her tune got remixed by Miley herself ... who stoked a conspiracy theory that sprung up during Brit's mental health facility stint.

The moment in question is almost as famous as the song itself -- Miley sings the refrain, "And a Britney song was on" three different times about midway through, but during Saturday's performance ... she shouted "Free Britney!" while her fans sang the normal lyrics from the crowd -- a clear indication that she's behind the #FreeBritney campaign.

As we've told you, a lot of Britney's fans were concerned she was being held captive at the mental health facility where she was getting treatment, under the control of the folks who run her lifetime conservatorship.

Britney herself tried to stamp out that notion by assuring her fans that she was fine, NOT being controlled by anybody ... and doing what was best for her at the time.