Metropolitan Police officers were called to Westow Hill, Crystal Palace, at around 3.39 am this morning (June 21) with reports of a stabbing.
A 30-year-old man was found suffering serious injuries at the scene and was treated before being taken to hospital.
His condition has since been confirmed to be neither life-threatening nor life-changing.
A short time later, officers located a second man with stab wounds in the Westow Hill area.
The man, also believed to be in his 30s, was pronounced dead at the scene.
The 30-year-old injured man has been arrested on suspicion of murder.
Det Chf Insp Sarah Lee said: “Our officers responded quickly alongside the London Ambulance Service who did everything possible to help the victim.
“This is a fast-moving investigation and residents in Crystal Palace will continue to notice an extensive police presence today as our enquiries continue.
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“There are a number of scenes in place, and I encourage anyone who may information or concerns to contact us.”
Anyone with information or footage is asked to contact police on 101, quoting CAD 1302/21JUN26.
Alternatively, information can be provided anonymously to Crimestoppers by calling 0800 555 111.