The incident happened in Rainham Road South near Aldborough Avenue at 8.25am today (January 15).
Officers and London Ambulance Service crews attended and treated a man before he was taken to hospital.
His condition is currently unknown.
Police enquires into the circumstances are ongoing.
A spokesperson for Metropolitan Police said: “Police were called on Wednesday, 15 January at 08:25hrs to an address on Rainham Road South, Dagenham where they found an unconscious man in his 30s.
“The London Ambulance Service attended and treated the man at the scene before he was taken to hospital. We await an update on his condition.
“Enquires into the circumstances are ongoing.
A spokesperson for London Ambulance Service said: “We were called at 9.23am today (15 January) to reports of a person unwell on Rainham Road South, Dagenham.
“We sent resources to the scene, including an ambulance crew, an advanced paramedic and an incident response officer.
“We treated a patient at the scene and took them to hospital.”
