Friends, family and community members gathered Sunday night in Lake Elsinore, California to honor and mourn Celeste Rivas ... the teen whose dismembered body was discovered in the trunk of a famous singer's car.

As you can see in the video ... people left messages, balloons, lights and candles for the 15-year-old ... who was reported missing prior to her body being discovered in a Tesla connected to D4vd .

We broke the story ... the body was identified as Celeste ... her mom telling us she went missing last year and had a boyfriend named David.

The Los Angeles County Medical Examiner's Office said the body was "severely decomposed" and appeared to have been in the car for an extended period of time before it was found.

Cops were looking into the possibility the teen was kil

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