Officers are appealing for witnesses after a collision in Gants Hill and an alleged subsequent assault on a woman.
Police were called at 18:53hrs on Friday, 4 January, to reports of a collision between two cars at Gants Hill Roundabout in Redbridge.
Officers attended and found the female driver of one of the cars, a Kia Picanto, taken unwell.
Officers immediately provided first aid prior to the arrival of the London Ambulance Service.
The 39-year-old woman has been taken to an east London hospital where she remains in a critical condition.
Officers are investigating a report that the woman was assaulted by the male driver of the second car, a Toyota Yaris, shortly after the collision.
The 54-year-old man has been arrested on suspicion of assault. He has been taken to an east London police station where he remains in custody.
Detectives are appealing for anyone who witnessed the collision or its aftermath to call the East Area Command Unit via 101, quoting CAD 5786.
In particular, officers wish to speak with a witness who was at the scene. He is a white man and around 6ft 2ins in height.
To remain anonymous, call Crimestoppers on 0800 555 111.