Officers in a marked Metropolitan Police vehicle attempted to stop a car on Long Lane, at its junction with The Glade, on the border of Addiscombe and Elmers End, at 1.51am on Tuesday (September 24).
The black car failed to stop and crashed into a road sign and pedestrian railings before overturning.
The London Ambulance Service and London Fire Brigade were called and a 21-year-old man was rescued from the overturned vehicle.
He was taken to a South London hospital where his condition is described as “critical”.
Images taken by witnesses show the severely damaged black vehicle on its roof on Long Lane.
The railings next to the car are “smashed to bits”, says one witness, while a road sign has also apparently been damaged.
Police have cordoned off Long Lane near to its junction with The Glade and detectives can be seen gathering evidence.
Scotland Yard has said the injured man’s next of kin have been informed of his condition and confirmed no one else was injured in the crash.
Two bus routes are being diverted due to the collision, while traffic is extremely heavy in the area. Long Lane remains closed.
One witness told MyLondon the scene of the crash “looks shocking”.
Another on Twitter added: “Someone’s absolutely stacked it on Long Lane by the junction with The Glade. Car on roof, lampposts and railings everywhere. Hope they are ok. Obviously the rain was not suited to their driving style.”
The Met’s Directorate of Professional Standards have been informed, Scotland Yard confirmed.
“This incident has now been referred to the Independent Office for Police Conduct (IOPC) who have declared this an independent investigation,” a spokesman for the force said.
When asked if the car was being chased by police when it crashed, the Met said “this will form part of the investigation”.