2:00 PM PT -- A forlorn Diddy was spotted sitting next to Kim's gravesite looking deep in thought. Several mourners -- including Usher -- later gathered on the cemetery lawn to watch the fireworks show for Kim.

Meanwhile, Diddy shared a touching tribute to her on his Instagram with the caption ... "The truest definition of a MOTHER."

Diddy and hundreds of other mourners have filed out of Kim Porter's funeral service at the Cascades Hills Church in Columbus, Georgia ... and family and friends are en route to her final resting place.

The procession is heading to Evergreen Memorial Park, where Kim will be buried next to her mother. We're told the fire department is already there because fireworks will be shot off in her honor, and police are on hand for security.

Diddy spoke at Kim's service along with many others -- including her son Quincy, Mary J. Blige and Dallas Austin ... while Yolanda Adams and Faith Evans paid musical tribute. Bishop Noel Jones delivered the eulogy.

Kim's being laid to rest in a golden casket. We're told as the procession left the funeral home, Marvin Gaye's "What's Going On" was playing.

TMZ broke the story ... Kim was found dead at her home last week after her family members say she was battling an illness.

She was 47.