Instagram video shows Cyrus trying to steal the show at the edge of the stage. Other stars in attendance included Selena Gomez and Katy Perry.
The 32-year-old pop singer's new 90-minute show is featured on The Strip at Planet Hollywood. She has signed on to perform 100 shows over the next two years.
The show is described as being visually similar to Cirque Du Soleil and includes acrobatics, a ring of fire, confetti, a live band, frenetic costume changes and a frequently airborne Spears.
It was speculated ahead of the show that Spears would not be singing live. Officials with Planet Hollywood parent company Caesars Entertainment Corp. have denied those claims.
Entertainment Programming Vice President Kurt Melien said Spears sings against a recorded track of her vocals to help her through the more physically grueling parts of the set, and add the electronic undertones that characterize many of her songs.
The Associated Press contributed to this report.