Celeste Rivas ' family and friends gathered Monday at Queen of Heaven Cemetery & Mortuary ... as they carried the slain 14-year-old’s casket to her final resting spot.

A hearse pulled up to the Rowland Heights, California cemetery, next to a large tent.

The funeral service will finally give her family and friends a chance to say goodbye ... after a case that’s rattled L.A. and left more questions than answers. TMZ.com

As we reported ... Celeste’s decomposed body was discovered last month in the front trunk of a Tesla belonging to singer D4vd ... the car was abandoned near a house the singer was renting in the Hollywood Hills and was impounded. Celeste's body was found days later in the vehicle at a Hollywood tow yard. TMZ.com

Police believe Celeste -- who friends in D

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