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Gracie Abrams brought down the house Thursday night in Los Angeles ... with some of Hollywood's youngest stars eating it all up.

It all went down at the Kia Forum -- part of her "Secret of Us" tour -- where she poured her heart out to a hometown crowd ... with celebs like Paul Mescal , Lucy Hale , Noah Beck , Tyler Blackburn , Lola Tung in the audience.

Paul is seen near the front row with a big smile on his face, clapping and singing along as Gracie performs her hit "Free Now."

Lucy Hale and her former "Pretty Little Liars" costar Tyler Blackburn snapped a smiling selfie in the crowd together, posing in matching white tops.

Noah Beck is looking suave ... chopping it up with "The Summer I Turned Pretty" star Lola Tung while rocking a backwards hat, as t

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