Watch Miley Cyrus shout 'Free Britney!' during 'Party in the U.S.A.' performance

It's not a party in the U.S.A. until Britney Spears is free, according to Miley Cyrus.

The singer-songwriter showed support for her friend and collaborator at a special Fourth of July concert in Las Vegas over the weekend, breaking from the lyrics of her 2009 hit "Party in the U.S.A." to call for an end to Spears' controversial legal conservatorship.

"Free Britney!" Cyrus shouted during the performance, replacing lyrics about Jay-Z with a shout-out to Spears (whom she also references in the words to the original track).

Cyrus has long been open about her fondness for Spears, and the pair even worked together on the former's 2013 album Bangerz, for which the pop icon recorded guest vocals on the song "SMS (Bangerz)."

In a recent court appearance, Spears spoke out against her father, Jamie Spears, and her ongoing legal conservatorship, detailing years of mistreatment and debilitating evaluations that she says has prevented her from living a normal life.

"I want changes, I deserve changes. I was told I have to be sat down and evaluated again," she said in her address, which later prompted resignations from her longtime manager, Larry Rudolph, and attorney, Sam Ingham. "I didn't know I could petition the conservatorship to end. I honestly didn't know that."

Watch Cyrus perform "Party in the U.S.A." in support of Spears in the fan-recorded video from the grand opening of the Resorts World casino above.

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