The trial was brought by Marcus Gray, better known as Christian rapper Flame, against the Grammy-nominated singer.
Katy Perry arrives in Los Angeles courthouse for copyright trial
He alleges she plagiarized his gospel song "Joyful Noise'' in her 2013 smash hit "Dark Horse.''
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Gray is expected to bring in a musicologist expert, who says the underlying beat in both songs is nearly identical.
Meanwhile, Perry's experts have submitted reports stating there is no similarity beyond generic elements.
"Joyful Noise'' hit YouTube in 2012 and has since been viewed nearly 2 million times.
The trial is expected to last about two weeks.