Police were called to the former Royal Hotel on Barkly Street in Footscray after a 37-year-old woman was found dead shortly before 2am.
The young girl was arrested at the scene.
She was taken to hospital and remains under police guard.
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A crime scene has been established as homicide and forensic detectives investigate the incident.
Anyone who witnessed the incident, has dashcam/CCTV footage or information is urged to contact Crime Stoppers on 1800 333 000.
