A 6-month-old baby girl was found dead in a dumpster in South Los Angeles ... and her mother is in custody as police investigate the infant's death as a homicide.

The baby was discovered in the trash receptacle late Monday night in South L.A. ... though LAPD says the child had been left there sometime in the previous 24 hours.

Cops say firefighters responded to the scene and took the baby to a nearby hospital ... where she was pronounced dead.

The LAPD tells TMZ ... officers received a call about a death investigation late Monday for an infant female found in a trash can.

LAPD's Juvenile Division is investigating the child's death as a homicide. Officers have already determined the child was not homeless, and the crime is not gang-related.

ABC7 reports they had a journalist on the scene who saw the baby's mother placed in handcuffs and taken away in a police vehicle.

The outlet also reports they spoke to a couple who said the baby might be their granddaughter. They claimed the daughter gave their grandkid to the child's father around 6 PM Monday, and the family hadn't heard from him.

Story developing ...